Support BCP-47 language fallback in translateQuestionnaire

[dmitryashutov]

Summary

translateQuestionnaire currently matches translation extensions with an exact language code comparison. FHIR Questionnaire.language and the translation extension lang both use BCP-47 codes from the Common Languages value set (http://hl7.org/fhir/ValueSet/languages), so values like de-AT, en-GB, fr-CA are valid.

When a region-specific language is requested (e.g. de-AT) but only a more general or related translation exists (e.g. de or de-DE), translation should still apply via a defined fallback chain instead of failing silently.

Proposed fallback order

Given a requested language (example: de-AT), resolve a translation in this order:

1. Exact match — de-AT
2. Primary language tag — de (strip region/script subtags)
3. Any other regional variant of the primary language — de-* (e.g. de-DE, de-CH)
4. English — en
5. Any English regional variant — en-* (e.g. en-GB, en-US)

If none of the above are present, leave the original text unchanged (current no-match behavior).

Examples

Requested	Available translations	Expected pick
de-AT	de-AT, de, en	de-AT
de-AT	de, en	de
de-AT	de-DE, en	de-DE
de-AT	en-GB	en-GB
de-AT	fr	(no change)
en-GB	en, de	en
en	en-US, de	en-US