Integrate #5466 Fix locale thousands separator producing '?' on Linux with multibyte UTF-8 separators

[marchcat]

Description

The original PR #5466 got messed up and was closed.
Clean cherry-pick PR #5550.

Test plan

• See Fix locale thousands separator producing '?' on Linux with multibyte UTF-8 separators #5466 for test guidance.

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[jellyfish-linden]

The test plan is unclear to me. I set LC_NUMERIC to nb_NO.UTF-8 before launching the viewer, but what exactly should I look for? What was the error we were seeing?

[marchcat]

You can likely see that in the "Develop -> Simulate a memory leak" floater, "Current leaked memory: N KB".
With nb_NO.UTF-8 it should not look like 1?024 KB.

[jellyfish-linden]

Is that the best way to do this? Is there nothing visible in viewer that exhibits this problem?

[marchcat]

If we need to check exactly that function, yes, it's the fastest one.
Alternative test would be "Build -> Pathfinding -> Region Objects", but that region should have enough objects (more than 999).

[jellyfish-linden]

Passed QA.
Verified on Second Life Release 26.4.0.34502396632 on Linux in the scope of https://github.com/secondlife/iqa/issues/604