@André-Programmeren said: As far as I remember I stopped and re-started Notepad++ Notepad++ saved it’s current language for that file (plain text) in the session config, then used that saved session config when you restarted. (This is a critical feature when one extension can have multiple meanings, and you have overridden the default language for a particular file) OTOH, if you close the file, Notepad++ discards meta-information, like what language was previously served, and so re-applies its internal rules and will see the UDL extension when you re-open the file.