Incorrect locale environment variable handling on Linux

[VorpalBlade]

Short description On Linux the LANG, LC_MESSAGES etc environment variables determine locale settings to be used. It appears rawtherapee only uses LANG. The correct behaviour is that:

• LC_ALL overrides everything if set
• LC_* (i.e. LC_MESSAGES, LC_CTYPE, etc) is used for the respective context if set
• LANG is used as a fallback.

However, rawtherapee completely ignores LC_MESSAGES and just uses LANG instead for the text in the GUI. I have not checked if this also applies to LC_NUMERIC, LC_TIME etc (this could affect things like the decimal separator).

Steps to reproduce

1. Execute in a shell:

   export LANG=sv_SE.UTF-8
   export LC_MESSAGES=en_US.UTF-8
   rawtherapee

2. Notice that menus are translated to Swedish, even though they should be in English. LC_ALL should also override, but doesn't. Note: Swedish translation is incomplete (which is yet another reason I don't want to use it), so some text will be in English.

Expected behavior The various locale environment variables should be respected so that people who prefers an English GUI but other things (such as time & date formatting, decimal separators etc) in their native locale get the correct experience.

Additional information

• Version of RawTherapee: 5.8 (tested version from Arch Linux package & flathub)
• Version of your operating system: Arch Linux (rolling release distro): Linux desktop 5.14.7-arch1-1 #2 SMP PREEMPT Thu, 23 Sep 2021 12:28:23 +0000 x86_64 GNU/Linux

Screenshot showing program being translated when it shouldn't be:

[innir]

Yes, true, RawTherapee only checks for LANG, see https://github.com/Beep6581/RawTherapee/blob/dev/rtgui/multilangmgr.cc#L142-L153 and https://github.com/Beep6581/RawTherapee/blob/dev/rtgui/multilangmgr.cc#L252-L258

Correct behavior would be to check for LC_ALL first, then for LC_MESSAGES and fallback to LANG if neither LC_ALL nor LC_MESSAGES exist.

NB: The decimal separator is forced to dot regardless the local.