The File Notes app is an IMatch app running in Chromium inside the app panel.
This is unrelated to IMatch's own spell-checking engine used i.e. in the Metadata Panel or Attributes panel, which is based on hunspall (also the default spell checker in Chromium so far).
I did not even know that Chromium somehow added or enabled (?) spell checking. Strange.
I see there is a right-click menu for correcting typos, but as far as I can see no way to select a language.
IMatch sets the language configured for the IMatch UI as the default language for Chromium and all apps.
This seems to be something Microsoft has added to the Chromium project, according to
https://blogs.windows.com/msedgedev/2020/05/28/improved-spellcheck-microsoft-edge-83/Maybe this somehow also affects the Chromium browser used by IMatch. I can only spell check in English, although I have enabled both English and German in Edge.
So far i could not find anything in the Chromium developer documentation on how to turn this on or off or how to allow the user to select a language.