From what Condottioero says it sounds like Modern Basque is basically a constructed new language with words based from every dialect rather than basing it off an existing dialect like you mentioned here with German. I guess it might be comparable to modern hebrew, which has been constructed based on various dialects and loanwords? at the same time it's probably based mainly around old hebrew. So who knowsModern German is based off of a literary dialect, that doesn’t mean that Modern German is invented, it was just based off of a dialect that most people didn’t traditionally speak. Standardization is a weird thing.