Have you tried HeliBoard? I think it has the best multilingual support for all these FOSS keyboards, but mind you, you have to manually load the dictionaries for the languages you want, and same thing for the swipe libraries, but it's all documented and linked on the GitHub page.
You have to configure the space bar to allow a long-press to give you a popup menu to select another language. Currently there's no way to have multiple languages with the same layout, it's annoying but you can work with it.