Hi
I would say "I'm going on holidays" plural in English. It might be regional, I'm Australian and have lived in London for 20 years. I think it could be used singular or plural.
I've been a bit confused because I would put it the same way as in Spanish. I've got a lot wrong thinking it was different. Could It be explained some non-US English call it holidays plural too. Are there any others that found this confusing?