I got the error with Tor Messenger 0.5.0b1 on Debian 9.1 (stretch) x86_64. When I installed the system, I had chosen the Polish language option. I ran the program like this:
$ ./start-tor-messenger.desktop
Note that, after the error appears, Tor Launcher will go ahead and bootstrap invisibly and the Tor Messenger UI will appear eventually. After that, the error will no longer appear, because Tor Launcher won't show the configuration dialog by default (it goes straight to the bootstrap progress bar). To force the error to happen again, you can do:
The Tor Launcher interface just appears in Polish. So it could be some other setting that got set when I made the language selection at system install time.