Fix gettext issues on Linux

[cameronwhite]

Description of Changes

• Fix the gettext library name for Linux, where these functions are provided by glibc rather than a separate libintl
• Add a unit test for translations. I'm not sure yet why this test fails on Ubuntu, but when running Pinta interactively the translations work fine.
• For extra safety, be more careful about not overriding the user's LANGUAGE environment variable.

Fixes: #2274

Checklist

• This pull request follows the project's contribution guidelines

[Skif-off]

Works fine.

Forgive my curiosity, why LANGUAGE instead of LANG?
The LANG and LC_* values ​​look the same, but the LANGUAGE value is a little strange:

$ locale
LANG=ru_RU.UTF-8
LANGUAGE=ru_RU:en

:en ???

[cameronwhite]

great, thanks for testing!
I'm not an expert on this, but I think ru_RU:en is specifying a sequence of fallback languages to use depending on what translations the application has

[Skif-off]

I tried LANG=en_US ./pinta and LANGUAGE=en_US ./pinta: in the first case, I got the message

(process:7478): Gtk-WARNING **: 12:32:13.256: Locale not supported by C library.
Using the fallback 'C' locale.

LANGUAGE looks more appropriate, will need to be remembered.

[cameronwhite]

yep, I think LANG affects more than just the display language, e.g. date/time formats, character encodiing etc, so you might need to have the locale / language pack installed on your system for those languages.