[11.x] Use correct pluralization rules in trans_choice for fallback strings

[stefanvdlugt]

The trans_choice function uses the pluralization rules of the preferred language. If the preferred language and the fallback language have different rules, and the key is not found in the preferred language, the fallback language is used but with the pluralization rules of the preferred language, causing unexpected results. See issue #52341 for an example.

This PR updates Translator::localeForChoice() so that it returns the fallback locale if the preferred language does not have a translation for the given key.

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[driesvints]

Thanks! @stefanvdlugt best to add a thorough explanation to your PR instead of just linking to an issue (see the PR template when you submit PR's).