It took me a while to understand the question, but I'd say that if you don't know how many painters are coming, you can just take your best guess as to singular or plural. This should be the similar to Spanish. If there's no reason to think there is only one painter, it would make sense (based on the real world of house painting, not on grammar rules) to say "they're going to paint" rather than "someone is going to paint". In real life, this is a great use for the passive voice "the buildings are going to be painted".

No