When I set my Test Language to Spanish on the Plugins tab, and then go to the Test tab and Start: Local Server, then the resulting Player windows do not have Spanish set as the Test Language.
This makes testing StarterPlayerScripts that rely on the language setting difficult to test and debug. I would expect that the Player windows should have the same language as I specified in the Plugins tab. I can manually set the language in the Player window, but this is after any startup scripts have already run.