In my google doc the spell checker has issues with words that have umlauts and gives me for every word an error:
I cannot reproduce that - how did you originally get that text into Google Docs, did you maybe paste it there? Could you copy a short section of the document to a new document and see if it still happens?
it looks like a transcoding error. (The plugin makes no attempt to translate Google Docs’ character encoding to the one used by LT, resulting in GIGO-failures)