Add UI language selection menu

[vitasnay-droid]

This PR adds a simple persistent UI language selection menu.

Changes:

• adds a uiLanguage setting
• loads the selected translation at startup
• adds a Language menu in the main window
• keeps language changes restart-based for simplicity and safety

Behavior:

• the selected language is saved in settings
• the language is fully applied after restart
• System default preserves the existing locale-based behavior

Additionally, this branch includes initial Russian (ru_RU) translation support.

[hmatuschek]

There is a common way to set the locale temporarily via environment variables. So I do not see the necessity to set the locate within the application.

Also, please add new translations via https://translate.codeberg.org/projects/qdmr/graphical-user-interface/

[ua0lnj]

I tried latest git.
lnj@ua0lnjhome:~$ export|grep LANG
declare -x GDM_LANG="ru_RU.UTF-8"
declare -x LANG="ru_RU.UTF-8"

But no russian language anywhere.

[hmatuschek]

Sorry, forgot to add translation to the resources.

[hmatuschek]

A call like

LANG=ru /your/path/to/qdmr 

works for me.