fix: main communication language to accept english-alone client-side

[yi171]

Description

fix for english alone to be accepted as the main communication language on client side
redo of closed PR fe#849 to use updated ISO codes instead of titles strings.

Related Issues

Closes #894

Changes

• update mainCommunication Zod validation in opportunityDetailsSchema.ts to accept German and/or English.
• update mainCommunicationInvalid error message in both en and de translation files

[nadavosa]

Hey @yi171 here is a review :

Review

Small, well-scoped fix: relaxes the client-side mainCommunication validation in opportunityDetailsSchema.ts to accept English-alone, matching the backend rule already relaxed in be#786. The new isOnlyGermanOrEnglish check correctly covers all three valid cases (German alone, English alone, both) and still rejects anything else, and it matches the fix prescribed in #894 essentially verbatim. Translation copy updates read correctly in both locales.

Nit

getMainCommunicationLanguageOptions (a few lines above the changed code, same file) has a comment that's now stale:

// The org's main communication language is German, with English as the only
// secondary option — unlike "Residents speak", which allows any language.

This describes exactly the old, now-relaxed rule ("German mandatory, English only as an addition"). Worth updating to reflect that German and English are each independently valid, together or alone, so a future reader doesn't get pointed at the wrong invariant.

Test coverage

No test exercises the three cases from #894's acceptance criteria (English-alone accepted, German-alone/both still work, a non-German/English pick still rejected) — worth a quick unit test on createOpportunityDetailsSchema's mainCommunication validation given this is the second attempt at this exact fix (the first, fe#849, was closed by mistake) and a regression here would be easy to miss without one.

Nothing blocking — logic is correct and matches the already-relaxed backend.