I'm not sure how to switch dictionaries other than when you install FileMaker, it gives you a choice of languages and the install sets things up according to that language.
You can turn off the under lining by going to File and pulling down to File Options and selecting the "Spelling" tab and unblock the "indicate questionable words with special underline". That takes care of things at the solution level. You can also turn it off and on with the inspector at the field level in case you want it to work sometimes and not others.
FYI, under File Options, you can not only turn underlining off and on, you can have it beep too. So if you want to drive someone nuts, turn on the beeping so all typos result in an audible!
Many thanks for your explanation on how to turn underlining off; I am novice in FileMaker and didn't know this. In any case, I think it would be very important that the program would allow:
1. Preferably to import or build main dictionaries of other languages which are not in the program now.
2. At least to turn all main dictionaries off, in order the user could correct the spelling using only the "User Spelling Dictionary".