Update TRANSLATING.md now that qm files are built automatically.

[softins]

Short description of changes
Updates the instructions in TRANSLATING.md to remove references to lrelease and .qm files, now that #3288 provides for the latter to be built automatically. Also add a step to the procedure for adding a new language.

CHANGELOG: Documentation: updated TRANSLATING.md for current build process

Context: Fixes an issue?
Fixes #3310

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

Status of this Pull Request
Ready for review

What is missing until this pull request can be merged?
Nothing

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• 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]

Thanks!

[softins]

Actually I need to make a couple of changes before merging. It's not necessary to stash the new language file if it has not yet been added to git, and in fact trying to probably wouldn't work. I'll check.

[softins]

Actually I need to make a couple of changes before merging. It's not necessary to stash the new language file if it has not yet been added to git, and in fact trying to probably wouldn't work. I'll check.

Update now added. Should be ready to merge.