The german dictionary in ID is weird. Many common german words it would mark as unknown, but the suggestions show a lot of rarely used ones. Well, when adding words to the dictionary, it can be confusing for the grammar checker to distinguish between verbs, adjectives and nouns if there is no definition for the word type. So, at least for German, but perhaps generally, give the user the option to define if the added word is a noun, adjective or verb.

Example for confusion: "brücken". This is a verb ("brücken" -> to bridge), but also the plural of "Brücke" (bridge), but with lowercase. The grammar checker would suggest to capitalize the word when it's meant to be a verb. I would then define it once as verb in lowercase and once as noun with capital letter in the dictionary.