Translation:We do not know what grandpa said.
Don't blame all your troubles on us Americans! Grandad is an acceptable word in American English, too. It's just a lot less common than Grandpa and Granddad. According to Ngram, Grandpa is more common than Grandad and Granddad (in that order) in British English, too, although by a smaller margin.
The moderators for DL Italian are volunteers and are severely understaffed, so it may a long time for your report to be addressed. But they're not trying to dismiss British usages as bad English. They just haven't gotten around to coding them in.
you have a main clause = "non sappiamo" = we don't know (sappiamo = verb "sapere" (to know) in the present tense,first person plural = we; it's the same as: Noi non sappiamo")
you have a subordinated clause = "cosa abbia detto il nonno." = what grandpa said; ("cosa" (as also its synonyms "che" and "che cosa") = what are used in questions [Cosa (Che/Che cosa) ha detto il nonno? = What did grandpa say?] and also in indirect sentences "what grandpa said"; (abbia detto is past tense in the subjunctive mood (the subjunctive is used (for example) when the main clause expresses a doubt or an uncertainty as in this case (we do NOT know), third person singular so "abbia detto" = (he/she/it) has said or said and then you have to translate the subject (who said something) = grandpa.
We don't know what grandpa "said" or "has said".
For grammar you can see the links on Marziotta's FAQ list:
Hello Veelysium, It's not a question you can answer through one sentence. Maybe, you will find your to some answers following this link. https://languagesandlinguistics.tumblr.com/post/95467130058/in-italian-are-there-contractions-like-you