Warn when the UI language's system locale isn't installed

[dsblank]

Motivation

Currently, gramps displays the following when a language pack is not installed:

(Gramps.py:2401429): Gtk-WARNING **: 05:37:58.349: Locale not supported by C library.
	Using the fallback 'C' locale.

We add the following translatable text:

.gramps.gen.utils.grampslocale.WARNING: Gramps could not find the system locale 
fi_FI.UTF-8 needed for the language fi. Dates, numbers, and sorting may not display
correctly until it is installed. See 
https://gramps-project.org/wiki/index.php?title=Gramps_6.1_Wiki_Manual_-_Setting_up_language_environment 
for instructions.

Note: That URL doesn't exist, but I couldn't find a page that would be helpful. We should remove the URL or fill that page in.

Summary

• Gramps resolves the chosen UI language to a full system locale (e.g. fi_FI.UTF-8) and writes it into LANG for Gtk to pick up.
• If that locale isn't actually installed, Gtk's own setlocale(LC_ALL, "") call silently falls back to the 'C' locale, printing only a generic, unhelpful warning: Locale not supported by C library. Using the fallback 'C' locale.
• GrampsLocale now probes for this ahead of time (_locale_is_installed()), and once translations are available (they aren't yet this early in construction), logs a translatable warning naming the missing locale, the language that needed it, and a wiki link with instructions on how to install it.

Test plan

• black --check on changed files
• mypy on changed files
• python3 -m unittest gramps.gen.utils.test.grampslocale_test — all passing, including new LocaleIsInstalledTest cases
• Manually reproduced: selecting a language whose locale isn't installed now logs Gramps could not find the system locale fi_FI.UTF-8 needed for the language fi. Dates, numbers, and sorting may not display correctly until it is installed. See <wiki-url> for instructions. instead of Gtk's opaque message; an installed language stays silent.

🤖 Generated with Claude Code

[sam-m888]

https://gramps-project.org/wiki/index.php?title=Gramps_6.1_Wiki_Manual_-_Setting_up_language_environment
for instructions.
Note: That URL doesn't exist, but I couldn't find a page that would be helpful. We should remove the URL or fill that page in.

I've added a stub section on the settings page in the 6.0 user manual that already has a number of sections on languages that can be cleaned up and simplified later on for the 6.2 release. I've also changed the section name from Setting_up_language_environment to Setting_up_the_language_environment

https://gramps-project.org/wiki/index.php/Gramps_6.0_Wiki_Manual_-_Settings#Setting_up_the_language_environment

When I roll over the user manual for 6.1 the link will work correctly and be available for the 6.2 release as

https://gramps-project.org/wiki/index.php/Gramps_6.2_Wiki_Manual_-_Settings#Setting_up_the_language_environment

What information should go into that section please, has it been written?

I've created a windows section and duplicated the existing message for windows installs that don't install all the languages

More information can be found at Installing_Gramps_for_Windows_computers#Missing_other_languages page