What did I miss by not speaking Dutch?
Probably quite a lot, especially if you'd been in the countryside.
Polish and German are useless, but with English and French
Polish is useful to me in Poland. Even very since at home we speak only Polish and most people I know socially in PL are not English speakers. Lucrative too since I translate professionally including best selling novels which can be a nice cash injection.
I've only used German in Germany (apart from chatting with elderly Germans in Spain). You are right; German is largely useless. It is a minor language.
Having good French though has helped me at, opening up work opportunities and social interactions in French-speaking countries. Plus, it's been handy watching films and reading.