RemNote does not have its own spell-checker – instead, it uses the spell-checker provided by your web browser (on the web app) or operating system (on the desktop app or mobile app). The language used by the spell-checker is thus determined by the settings in your browser or operating system.
For your web browser, you can usually tell the browser what languages you know and which you want it to spell-check. For example, in Google Chrome and other Chromium-based browsers, you can find the option under “Languages” in the settings.
For your operating system, you can usually only configure one system language, and this will also control the language that interface text appears in. The details will depend on which operating system you're using. (If this makes using the desktop app a pain, you could consider using the web app instead.)
If you're unable to set your browser or operating system language to match the language you're taking notes in, for whatever reason, and you'd prefer having the spell-checker turned off to having it check your notes in the wrong language, you can disable it in Settings > Editor > General.
