How to do so depends on which language you want to translate for. Updating one of the existin languages should be easy, but adding a new language would require manually ensuring the appropriate xml files are created.
The Bannerlord Online Localization Manager is just a visual tool to make it slightly simpler to translate everything, as the translations could also be done directly in the appropriate xml files.
It's important to note that whenever Bannerlord Online has any update, which is at least monthly, it will remove any of the changes you may have made and replace it with the default available localizations. Meaning you should store a copy of the files outside of the module folder if you want to preserve it.
If you want the changes to be included in BO, you should reach out to Moonlight and/or Storm through the discord. Thouhg replies may be slow.