Re-allow localisation when SOURCE_DATE_EPOCH is set (follow-up to #10949)

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• I may have take an entirely incorrect (basically, inverted) approach to solving LANGUAGE environment variable inconsistently affects output of objects.inv  #9778
• The approach - disabling localisation entirely for reproducible builds (Disable localisation when SOURCE_DATE_EPOCH is set #10949) is likely not a good idea
• I'd really appreciate sanity-checking on this

Detail

• Disable localisation when SOURCE_DATE_EPOCH is set #10949 fixed an unusual behaviour around lazy-loading of localised resources

  • During investigation, it turned out that many gettext-localized values -- particularly in Python modules under sphinx.domains -- were being translated at module-load-time (in other words, when each Python module was import'ed) -- and would not subsequently be re-localized.

• However, Disable localisation when SOURCE_DATE_EPOCH is set #10949 also completely disabled localisation during reproducible builds (detected by a non-empty SOURCE_DATE_EPOCH environment variable)
• Disabling localisation may not have been necessary: as mentioned in the original bug description, the objects.inv file -- where reproducibility problems were occurring -- was already being requested in a single language (English-only); so localisation of the rest of the package-under-build should be irrelevant

Relates

• Original bug report: LANGUAGE environment variable inconsistently affects output of objects.inv  #9778
• Previous fix attempt: Disable localisation when SOURCE_DATE_EPOCH is set #10949

[jayaddison]

Sorry @AA-Turner @lamby; I don't currently feel confident that #10949 was the correct fix -- and nor am I confident that this patch makes it correct.

I'm going to revert #10949 completely until I can confirm repro and fix of the originally-reported issue. Maybe these changes do resolve the problem, but for a large widely-used project, I think that a 'first, do no harm' principle makes sense.

[jayaddison]

Revert opened as #11306.