Docs: Document translation creation

Fixes: #2755

docs/TRANSLATING.md

@@ -262,6 +262,39 @@ git push --force # Push the changes to your repo. Be aware that this will overwr
 
 ---
 
+## Adding a new language (checklist)
+
+The following section describes how to add a new language and what to do or wait for.
+Please open an issue on GitHub and introduce yourself first, even if you intend to add the translation files yourself.
+
+After having opened an issue on GitHub:
+1. Fork the Jamulus repository as described above, then install Qt on your machine (See [COMPILING.md](../COMPILING.md)). (You will need `lrelease` and `lupdate` later on.)
+2. Create a branch based on the `master` branch: Locally navigate to the folder you cloned the repo (with `cd /path/to/jamulus/repo`) and type `git checkout -b "translate-add-xx_XX"` where xx_XX is the language code of the new language.
+3. Open Jamulus.pro in an editor such as `nano`: `nano Jamulus.pro` and add your language file (which will be created later) in alphabetical order to the `TRANSLATIONS` variable. The format should match the other present languages.
+4. Also add `src/translation/translation_xx_YY.qm` to the DISTFILES variable similar to the other files
+5. Save and close Jamulus.pro.
+6. Add the `.qm` file in `src/resources.qrc` similar to other languages.
+7. Now move back to the repository root and run `lupdate Jamulus.pro`. Afterwards, run `lrelease Jamulus.pro` which should create all of the necessary files.
+8. Finally, add, commit and push the changes to your repository with the following commands: `git add src/translation/translation_xx_YY.* src/resources.qrc`, `git commit -m "Translation: Add empty xx_YY translation"`, `git push`
+9. Open a Pull request from your repo to the main repo with a brief description that you plan to translate Jamulus into your language
+10. Once the pull request is merged, Weblate should pick up the new language.
+
+### Adding a translation to the Windows installer
+
+To add a new translation to the Installer, assuming you do similar steps with git as described above:
+
+1. Copy `src/translation/wininstaller/en_UK.nsi` to `src/translation/wininstaller/xx_YY.nsi`
+2. Add your language with [the correct language code](https://nsis.sourceforge.io/Examples/Modern%20UI/MultiLanguage.nsi) to `src/translation/installerlng.nsi` by following the file format
+3. Rename `${LANG_ENGLISH}` to the respective value you set in installerlng.nsi in xx_YY.nsi
+
+### After a language was added
+
+After adding a language, you can begin translating via Weblate or git, as described in the first section of this guide. If you intend to work on the translation for longer, you will be invited to the Jamulus organisation as well as translators channel on Discord or Matrix. To gain access, please contact the administrators.
+
+Because every contributor can and should be added to the in-app contributor list, please remind us to add you to the `src/utils.h` file as translator or raise a Pull request yourself.
+
+---
+
 ## That's all
 
 Thank you for contributing!