I have to say that with everyone always complaining about Siri I was pleasantly surprised by how good the dictation is on the watch.
I speak three different languages regularly. It seems like the watch knows what language the keyboard or the conversation in the message is in. So I can dictate in English in one message. In another message in another language and so on. It got all of them right without skipping a beat.
I was really very impressed with it.
I did notice that this only worked for iMessage. I tried it in Facebook messenger and it completely failed.