Document weblate: tools and TRANSLATING.md

[ann0see]

Short description of changes

Document Weblate in Translating.md and tools script. Also added adding a new language - although it's strictly speaking not weblate related. However, it's based on the changes, so adding it as separate commit.

CHANGELOG: Translation: Document translation process shift to Weblate. Translators can from now one use Weblate for translations.

Context: Fixes an issue?

Fixes: https://github.com/jamulussoftware/jamulus/issues/2758 (at least I think that's the last todo?)

Does this change need documentation? What needs to be documented and how?

No. Hopefully that's all

Status of this Pull Request

What is missing until this pull request can be merged?

Checklist

• [x] I've verified that this Pull Request follows the general code principles
• [ ] I tested my code and it does what I want
• [x] My code follows the style guide
• [ ] I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• [ ] I've filled all the content above

[ann0see]

@henkdegroot you might want to have a look at this as you commented on https://github.com/jamulussoftware/jamulus/issues/2758#issuecomment-1257176540

[ann0see]

I'd like to get this in soon.