gh-64243: Implement locale.getlocale fall back in gettext.find

[StanFromIreland]

LANGUAGES envvar is prioritized over getlocale for consistency with C gettext.

• Issue: gettext.install() ignores previous call to locale.setlocale() #64243

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📚 Documentation preview 📚: https://cpython-previews--131477.org.readthedocs.build/

[StanFromIreland]

request: @tomasr8

[StanFromIreland]

@serhiy-storchaka Could you please review this?

[serhiy-storchaka]

There are issues with using locale.getlocale(). It ignores the modifier and can raise ValueError. I suggest to use locale._setlocale().

Needed tests for locales "C", "C.UTF-8", locales with a modifier, locales with and without encoding.

To help testing, you can extract the code into the _guess_languages() function.

[StanFromIreland]

There are several existing tests (from a previous pr) for the C locale.

[StanFromIreland]

CIs don’t have some locale, I will have to add requirements.

[StanFromIreland]

@tomasr8 as the expert now, could I get a review:-)

[tomasr8]

I can have a look this weekend, though I'd still prefer to get a final signoff from Serhiy :)

[github-actions]

This PR is stale because it has been open for 30 days with no activity.