The mix up of languages happens to me in the waking world too. I'm a native German speaker currently living in the UK. English always felt much more natural to me, even back when I learned it in school. The more you use it the more it takes over, too. When I'm thinking or talking to myself (yes, I talk to myself out loud lol. Mostly when I'm trying to solve problems, it helps me to talk about them even if it's just myself I'm talking to) I tend to do that in English. Often when I speak German the English words pop into my mind first and I have to think what the right German words are. I've not had a lucid dream yet, but I've had a vivid one last night which I remember very well. In it I was speaking to an audience and I was doing so in English, not in German. I also speak French and I'm reading and hearing French often, but I'm not speaking it a lot, none of this has happened with French before.

So I believe it's a matter of what language you are currently used to and, more importantly, actively using. Speaking and writing is much more powerful in that regard than hearing or reading. I'm not sure what impact it has on lucid dreams to write your dream journal in a foreign language, but it's a great way to improve and polish your linguistic skills