Is Joplin allowed to access the Internet? When you enable spell checker for a specific language for the first time, it needs to download a dictionary for it in order to work.
They will be kept in a directory called Dictionaries inside the folder with Electron files. In Windows, the path is %APPDATA%\Joplin, but I'm not sure what the location in Linux is . Please keep in mind that this isn't the same path as the one where the database, resources, settings, etc. are stored.