First off, thanks for taking the time to contribute! 🎉
To test your changes in JOSM, from po directory, run make. This will create a zip file that can be loaded in the JOSM validator Preferences page.
You can set the parameter validator.check_assert_local_rules to true in JOSM Preferences page and launch josm from the command line.
You will then get errors in the console if JOSM fails to parse some rules or if the tests (assertMatch and assertNoMatch) do not pass.
If you have updated the labels inside the MapCSS file you need to update the translation strings : from po directory, run make *.po and fix or add new translations.
You don't have to provide all languages, the project is on transifex so translators can give us some help on that 👍
Don't forget to commit the .pot and .po files ;)