I do not know if grammarly supports the german language...
Grammarly only works with English and I think its default settings is British English, not American.
Most grammar and spell checkers use American English too so it might not be perfect if you are in a country that does not use american english as the spelling would be different for certain words.
Unless you are a language expert or a native that pays attention to that sort of thing, it's not something you will notice. Surely those that rely heavily on spellcheckers wouldn't mind which variant they work with. Your browser settings can be configured to use spellchecking for British or American English, so that's adjustable according to your needs and isn't a problem to change.