Hi!
This might be as well a problem with the LibreOffice itself and I’ve logged a bug report separately but wanted to also check here. Maybe someone has any insight?
With the current 7.4.x LO line (and the future 7.5.x line as well) there is an issue that occurs after the LanguageTool extension has been installed. Whenever there are two or more dictionaries active in LO/Writer and the the LanguageTool extension is present and active, it is impossible to add new words to any of the dictionaries.
I’ve been looking for a workaround and unfortunately the only solution is to uninstall LT extension. I tried remote functionality (introduced in LO 7.4: I can only check free API https://api.languagetool.org/v2), however, while the words are being added to a dictionary, they are not being recognized afterwards and are constantly marked as errors (regardless of the number of dictionaries active; this is yet another issue but less crucial for me as I prefer working with the offline instance of LT).