I'm really impressed that floating editor windows are available in the Insider builds, I'm definitely going to give it a try, I've been waiting for that for what now seems forever.
I have been using a French/English layout with Gboard for a long time without any problems.
The text prediction algorithms even support me changing language in the middle of a phrase.
It's been so long since I used SwiftKey, but I think the swipe + multi-lingual support was the reason I initially switched back to Gboard when they added swipe support...