Hello, welcome to the Community! 
The spelling and grammar routines are currently only available in English. There are some settings within Papyrus Author relating to use of other languages, e.g. you can mark your text as a different language, but this does not change the spelling and grammar check.
We are considering adding support for other languages in the future.
There is a topic that goes further into these settings here: Default language other tha English does not work