Hi all, I'm the Stellaris producer. Quick update:
For those on getting failure to launch on the stellaris_test branch on Linux, either opt out of that beta branch, or do the following:
Start Stellaris through the terminal, or a by adding the "-nolauncher" startup command. This can be done by right-clicking on Stellars in the steam library, click on properties and then click on Set Launch Options and enter "-nolauncher"
There's some weird config stuff going on. We are looking at more permanent solutions and will get those out as soon as possible, but here's a way you can play in the meantime.