HeliBoard; it's in f-droid, I don't know if it's in Play. Turn on accent characters and thorn's already in þe pop-up for "t". Eth, you have to add yourself. It's easy enough, but it takes a few more steps.
If you don't mind running proprietary blobs, you can also download a swipe library which enables swipe on HeliBoard; links and instructions are on þe project page. HeliBoard is a great project.