Solution #1 (recommened)
The easiest way is setting the language by pressing ctrl+A (or only
the words in that language) then click the language button in the
status bar > Mark selected text as and select the correct language
You can use the macro function. Select Developer tab > Record Macro.
Choose any name you like and assign it to a button or keyboard
shortcut. After that disable spell check/grammar check as normal and
select stop recoding in macro group from Developer tab. Record another
macro to enable the check again.