After I had merged some mods the localisation didn't work because the files were saved as UTF-8 instead of UTF-8-BOM as required per the wiki: https://stellaris.paradoxwikis.com/Localisation_modding#File_Encoding. After I fixed this the localisation worked again.
Please save localisation files as UTF-8-BOM
After I had merged some mods the localisation didn't work because the files were saved as UTF-8 instead of UTF-8-BOM as required per the wiki: https://stellaris.paradoxwikis.com/Localisation_modding#File_Encoding. After I fixed this the localisation worked again. Please save localisation files as UTF-8-BOM