It’s a known issue (not a bug) as the web app uses a library called hunspell which relies on a rather complicated dictionary file format for spell checking. The issue stems from the fact that the only available German dictionary has a license incompatible with the web app and contacting its author to purchase the dictionary has not yet led to any results unfortunately.
@xkevio is it be possible to expose an interface in the web app to bring a local dictionary or strip all the code and export the text only in order to check it externally? I write a lot in German, and this particular shortcoming makes it significantly harder to directly use typst for projects – I am stuck writing all the text first and then typesetting it in Typst.