Enable translations (internationalization/i18n)

As talked today during the meeting, we need to start implementing internationalization on the user facing strings.

An introduction for developers can be found at https://gitlab.torproject.org/tpo/community/l10n/-/wikis/Localization-for-developers and I am open for questions about the implementation.

From the localization maintenance POV, I like the gettext-po approach, but weblate supports also other formats: https://docs.weblate.org/en/latest/formats.html

We could start the implementation now, and once the strings are more ready we can send them for translation.