diff --git a/assets/js/app/editor/Components/Html.vue b/assets/js/app/editor/Components/Html.vue
index a17e5c85b0..63f7dfb49a 100644
--- a/assets/js/app/editor/Components/Html.vue
+++ b/assets/js/app/editor/Components/Html.vue
@@ -1,14 +1,92 @@
 <template>
     <div>
-        <trumbowyg :id="id" v-model="val" :name="name" :config="config"></trumbowyg>
+        <trumbowyg v-if="ready" :id="id" v-model="val" :name="name" :config="config"></trumbowyg>
     </div>
 </template>
 
 <script>
+import jQuery from 'jquery';
 import trumbowyg from 'vue-trumbowyg';
 import 'trumbowyg/dist/ui/trumbowyg.css';
 import { strip } from '../../../filters/string';
 
+// Trumbowyg's UI translations, one langs/<code>.min.js per language. In lazy mode each
+// file is its own chunk (trumbowyg-lang-<code>.js), fetched only when requested.
+// Loading a file registers it on jQuery.trumbowyg.langs. English is built in.
+const langFiles = import.meta.webpackContext('trumbowyg/dist/langs', {
+    recursive: false,
+    regExp: /\.min\.js$/,
+    mode: 'lazy',
+    chunkName: 'trumbowyg-lang-[request]',
+});
+
+// Trumbowyg names some languages differently from the ISO codes Bolt uses. Its `my`
+// file is meant to be Malay, while ISO `my` is Burmese, so that one must not match
+// as-is. Malay (`ms`) is not aliased to it either: the file registers itself as `id`.
+const langAliases = new Map([
+    ['be', 'by'],
+    ['fil', 'ph'],
+    ['my', null],
+    ['nn', 'nb'],
+    ['no', 'nb'],
+    ['sr', 'rs'],
+    ['sr_latn', 'rs_latin'],
+    ['tl', 'ph'],
+    ['uk', 'ua'],
+    ['zh', 'zh_cn'],
+    ['zh_hans', 'zh_cn'],
+    ['zh_hant', 'zh_tw'],
+    ['zh_hk', 'zh_tw'],
+    ['zh_mo', 'zh_tw'],
+]);
+
+// Returns the Trumbowyg language matching a Bolt locale, or null when Trumbowyg has no
+// translation for it. Tries the whole locale first, then drops one part at a time:
+// `sr_Latn_RS` -> `sr_latn` -> `sr`, `de_AT` -> `de`.
+function findLang(locale) {
+    const parts = locale.toLowerCase().replace(/-/g, '_').split('_');
+
+    for (let length = parts.length; length > 0; length--) {
+        const candidate = parts.slice(0, length).join('_');
+        const lang = langAliases.has(candidate) ? langAliases.get(candidate) : candidate;
+
+        if (lang && langFiles.keys().includes(`./${lang}.min.js`)) {
+            return lang;
+        }
+    }
+
+    return null;
+}
+
+// Loads the Trumbowyg language matching a Bolt locale and resolves to its code, or to
+// 'en' when there is no translation or it fails to load.
+async function loadLang(locale) {
+    const lang = findLang(locale);
+
+    if (!lang) {
+        return 'en';
+    }
+
+    try {
+        await langFiles(`./${lang}.min.js`);
+    } catch {
+        return 'en';
+    }
+
+    // A file that registers itself under another name would leave Trumbowyg without
+    // the language it was told to use.
+    if (!jQuery.trumbowyg.langs[lang]) {
+        return 'en';
+    }
+
+    // Arabic, Hebrew and Persian also set `_dir: 'rtl'`, which Trumbowyg applies to the
+    // editable area. The text direction belongs to the content being edited, not to the
+    // backend language, so only the labels are used.
+    delete jQuery.trumbowyg.langs[lang]._dir;
+
+    return lang;
+}
+
 export default {
     name: 'EditorHtml',
     components: {
@@ -19,9 +97,14 @@ export default {
         label: String,
         name: String,
         id: String,
+        locale: {
+            type: String,
+            default: 'en',
+        },
     },
     data: () => {
         return {
+            ready: false,
             val: null,
             config: {
                 btns: [
@@ -40,6 +123,12 @@ export default {
             },
         };
     },
+    async created() {
+        // Trumbowyg reads its language when it initialises, so it is only rendered once
+        // the language is set.
+        this.config.lang = await loadLang(this.locale);
+        this.ready = true;
+    },
     mounted() {
         this.val = strip(this.value);
     },
diff --git a/templates/_partials/fields/html.html.twig b/templates/_partials/fields/html.html.twig
index 7d34dc994d..09f8b14a31 100644
--- a/templates/_partials/fields/html.html.twig
+++ b/templates/_partials/fields/html.html.twig
@@ -5,5 +5,6 @@
         :value="{{ value|json_encode }}"
         :name='{{ name|json_encode }}'
         :id='{{ id|json_encode}}'
+        :locale='{{ app.request.locale|json_encode }}'
     ></editor-html>
 {% endblock %}
diff --git a/tests/unit/components/Html.spec.ts b/tests/unit/components/Html.spec.ts
index 83a03aecff..2783fb3de1 100644
--- a/tests/unit/components/Html.spec.ts
+++ b/tests/unit/components/Html.spec.ts
@@ -1,8 +1,10 @@
 import { mount } from '@vue/test-utils';
 import { nextTick } from 'vue';
-import { describe, expect, it } from 'vitest';
+import jQuery from 'jquery';
+import { describe, expect, it, vi } from 'vitest';
 import Html from '@/editor/Components/Html.vue';
 import { trumbowygStub } from '../helpers/stubs';
+import { contextLoader } from '../helpers/webpack-context';
 
 const defaultProps = {
     value: 'Some <strong>html</strong>',
@@ -68,4 +70,112 @@ describe('EditorHtml', () => {
         expect(config.btns).toContainEqual(['viewHTML']);
         expect(config.btns.flat()).not.toContain('undo');
     });
+
+    describe('toolbar language', () => {
+        type TrumbowygLangs = Record<string, Record<string, string> | undefined>;
+        const langs = () => (jQuery as unknown as { trumbowyg: { langs: TrumbowygLangs } }).trumbowyg.langs;
+
+        // Trumbowyg is only rendered once its language is known, and loading one
+        // is a dynamic import, so wait for the editor rather than for a tick.
+        const mountReady = async (props: Record<string, unknown> = {}) => {
+            const wrapper = mountHtml(props);
+            await vi.waitFor(() => expect(editor(wrapper).exists()).toBe(true));
+
+            return wrapper;
+        };
+
+        const lang = (wrapper: ReturnType<typeof mount>) => (editor(wrapper).props('config') as { lang: string }).lang;
+
+        it('stays in English without a locale, and loads nothing', async () => {
+            const load = vi.spyOn(contextLoader, 'load');
+            const wrapper = await mountReady();
+
+            expect(lang(wrapper)).toBe('en');
+            expect(load).not.toHaveBeenCalled();
+        });
+
+        it('loads the translation matching the locale', async () => {
+            const load = vi.spyOn(contextLoader, 'load');
+            const wrapper = await mountReady({ locale: 'cs' });
+
+            expect(lang(wrapper)).toBe('cs');
+            expect(load).toHaveBeenCalledOnce();
+            expect(load.mock.calls[0][0]).toBe('./cs.min.js');
+            expect(langs().cs?.viewHTML).toBe('Zobrazit HTML');
+        });
+
+        it('renders the editor only once the language is loaded', async () => {
+            const wrapper = mountHtml({ locale: 'cs' });
+            await nextTick();
+
+            expect(editor(wrapper).exists()).toBe(false);
+
+            await vi.waitFor(() => expect(editor(wrapper).exists()).toBe(true));
+            expect(lang(wrapper)).toBe('cs');
+        });
+
+        it.each([
+            ['de_AT', 'de'],
+            ['pt_BR', 'pt_br'],
+            ['pt-BR', 'pt_br'],
+            ['es_MX', 'es'],
+            ['sr_Latn_RS', 'rs_latin'],
+            ['sr_Cyrl_RS', 'rs'],
+        ])('falls back from %s to the closest translation, %s', async (locale, expected) => {
+            const wrapper = await mountReady({ locale });
+
+            expect(lang(wrapper)).toBe(expected);
+        });
+
+        it.each([
+            ['uk', 'ua'],
+            ['sr', 'rs'],
+            ['be', 'by'],
+            ['fil', 'ph'],
+            ['tl', 'ph'],
+            ['no', 'nb'],
+            ['nn', 'nb'],
+            ['zh', 'zh_cn'],
+            ['zh_Hans', 'zh_cn'],
+            ['zh_Hant', 'zh_tw'],
+            ['zh_HK', 'zh_tw'],
+            ['zh_MO', 'zh_tw'],
+        ])('maps %s to the Trumbowyg name %s', async (locale, expected) => {
+            const wrapper = await mountReady({ locale });
+
+            expect(lang(wrapper)).toBe(expected);
+        });
+
+        // Trumbowyg's `my` file is meant to be Malay, while ISO `my` is Burmese,
+        // and the file registers itself as `id`, so Malay has no usable file either.
+        it.each(['my', 'my_MM', 'ms', 'xx', 'en_GB'])('stays in English for %s, and loads nothing', async locale => {
+            const load = vi.spyOn(contextLoader, 'load');
+            const wrapper = await mountReady({ locale });
+
+            expect(lang(wrapper)).toBe('en');
+            expect(load).not.toHaveBeenCalled();
+        });
+
+        it('stays in English when the translation fails to load', async () => {
+            vi.spyOn(contextLoader, 'load').mockRejectedValueOnce(new Error('Loading chunk failed'));
+            const wrapper = await mountReady({ locale: 'fr' });
+
+            expect(lang(wrapper)).toBe('en');
+        });
+
+        it('stays in English when the translation registers no language under its name', async () => {
+            vi.spyOn(contextLoader, 'load').mockResolvedValueOnce({});
+            const wrapper = await mountReady({ locale: 'vi' });
+
+            expect(lang(wrapper)).toBe('en');
+        });
+
+        it.each(['ar', 'he', 'fa'])('uses the %s labels but keeps the editing direction', async locale => {
+            const wrapper = await mountReady({ locale });
+
+            expect(lang(wrapper)).toBe(locale);
+            expect(langs()[locale]?.viewHTML).toBeTruthy();
+            expect(langs()[locale]).not.toHaveProperty('_dir');
+        });
+    });
 });
diff --git a/tests/unit/helpers/webpack-context.ts b/tests/unit/helpers/webpack-context.ts
new file mode 100644
index 0000000000..c4553316df
--- /dev/null
+++ b/tests/unit/helpers/webpack-context.ts
@@ -0,0 +1,56 @@
+/**
+ * `import.meta.webpackContext()` for the test run.
+ *
+ * Webpack replaces the call at build time with a context over the matching
+ * files. Vite has no equivalent, so the `webpack-context` plugin in
+ * vitest.config.ts rewrites the call to this function, installed by setup.ts as
+ * `globalThis.__webpackContext`. The plugin lists the directory in place of the
+ * path it was given, as a map of `./<file>` to a lazy import of that file.
+ *
+ * Only the form the app uses is supported: a non-recursive, lazy context. Its
+ * keys are `./<file>`, as webpack's are, and calling it with one imports that
+ * file and resolves to the module.
+ */
+
+export type ContextFiles = Record<string, () => Promise<unknown>>;
+
+export type WebpackContextOptions = {
+    recursive?: boolean;
+    regExp?: RegExp;
+    mode?: string;
+    chunkName?: string;
+};
+
+export type LazyWebpackContext = {
+    (request: string): Promise<unknown>;
+    keys(): string[];
+};
+
+/**
+ * Imports one file of a context. A spec spies on `load` to see which file was
+ * requested, or to make it fail to load, as a chunk can in the browser.
+ */
+export const contextLoader = {
+    load: (request: string, importFile: () => Promise<unknown>): Promise<unknown> => importFile(),
+};
+
+export function webpackContext(files: ContextFiles, options: WebpackContextOptions = {}): LazyWebpackContext {
+    const { recursive = true, regExp = /^\.\/.*$/, mode = 'sync' } = options;
+
+    if (recursive || mode !== 'lazy') {
+        throw new Error('Only non-recursive, lazy webpack contexts are supported in the unit tests.');
+    }
+
+    const keys = Object.keys(files).filter(key => regExp.test(key));
+
+    const context = (request: string): Promise<unknown> => {
+        if (!keys.includes(request)) {
+            return Promise.reject(new Error(`Cannot find module '${request}'`));
+        }
+
+        return contextLoader.load(request, files[request]);
+    };
+    context.keys = () => [...keys];
+
+    return context;
+}
diff --git a/tests/unit/setup.ts b/tests/unit/setup.ts
index 49e31e974a..9269707d48 100644
--- a/tests/unit/setup.ts
+++ b/tests/unit/setup.ts
@@ -4,6 +4,7 @@ import $ from 'jquery';
 import { afterEach, beforeEach, expect, vi } from 'vitest';
 import { resetDom } from './helpers/dom';
 import { installWarningHandler, resetWarnings, reviewWarnings } from './helpers/warnings';
+import { webpackContext } from './helpers/webpack-context';
 
 Vue.config.productionTip = false;
 Vue.config.devtools = false;
@@ -14,6 +15,11 @@ Vue.config.devtools = false;
 (window as unknown as Record<string, unknown>).$ = $;
 (window as unknown as Record<string, unknown>).jQuery = $;
 
+// The target vitest.config.ts rewrites `import.meta.webpackContext` to. Html.vue
+// calls it when the module is first imported, so it has to exist before any spec
+// imports a component.
+(globalThis as unknown as Record<string, unknown>).__webpackContext = webpackContext;
+
 // Date.vue's updated() hook calls both on the flatpickr input. jsdom's
 // constraint-validation support is partial, so make sure they exist.
 if (!HTMLInputElement.prototype.reportValidity) {
diff --git a/vitest.config.ts b/vitest.config.ts
index 0e57c3207e..b2ae9980ac 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -1,12 +1,44 @@
 import { defineConfig } from 'vitest/config';
 import vue2 from '@vitejs/plugin-vue2';
+import fs from 'node:fs';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 
 const root = path.dirname(fileURLToPath(import.meta.url));
 
 export default defineConfig({
-    plugins: [vue2()],
+    plugins: [
+        vue2(),
+        // Html.vue builds its Trumbowyg language context with webpack's
+        // `import.meta.webpackContext()`, which Vite does not implement. Route
+        // the call to the stand-in in tests/unit/helpers/webpack-context.ts,
+        // which setup.ts installs, and replace the node_modules path it was
+        // given with a lazy import of every file in that directory.
+        {
+            name: 'webpack-context',
+            transform(code) {
+                if (!code.includes('import.meta.webpackContext(')) {
+                    return null;
+                }
+
+                const rewritten = code.replace(
+                    /import\.meta\.webpackContext\(\s*(['"])(.+?)\1/g,
+                    (_match, _quote, request: string) => {
+                        const directory = path.resolve(root, 'node_modules', request);
+                        const files = fs
+                            .readdirSync(directory)
+                            .map(file => `'./${file}': () => import(${JSON.stringify(path.join(directory, file))})`);
+
+                        return `globalThis.__webpackContext({ ${files.join(', ')} }`;
+                    },
+                );
+
+                // The call is rewritten in place on a single line, so line
+                // numbers, and with them coverage, are unaffected.
+                return { code: rewritten, map: null };
+            },
+        },
+    ],
     resolve: {
         // Components import each other without the extension (`from './_SubMenu'`),
         // which webpack resolves via its own `resolve.extensions`. Vite does not