Thank you for you time. About the UI language, I think I will need to ask it in Nextcloud forums.
About spell-checking: how to set a default language system wide? Everytime I open a document, English (USA) is the default one. Because my Nextcloud UI is already a different language but Collabora is still displayed in English.
No, only for updating I asked. Because the dictionary versions shipped with Collabora is a bit old.