Is it okay to switch languages in an email?
It would be weird to see an email in multiple languages, but it's not completely unusual or unheard of. It would certainly make me wonder what's going on if I received such an email, but it's not a breach of etiquette.

In my own personal experience, I had a lot of communications in German. In some cases, if someone was quoting something, or using terms that are hard to translate, they'd throw in the English and then keep on using German.