When launching Roblox from the website, client authentication may fail, showing a message box like this.
However, it’s not able to properly display localised text for languages other than English (Portuguese and Chinese pictured below):
To reproduce:
Set your system display language to one that’s not English
Trigger an authentication error with this protocol launch URI: roblox-player:1+gameinfo:a
This appears to be exclusive to the Windows client only.
(aside - it would be nice if this dialog could provide more user-friendly information, since the auth API is ratelimited and therefore this error will show if a player is joining games too quickly, and confuse the hell out of them)
This could be for many reasons. A main reason is one of your accounts may have gotten banned recently and Roblox is blocking you from using their applications. Just wait a little or clear your local Roblox temp folder on your PC and reinstall.
Ok, well consider looking over my other steps I gave to run Roblox again. Make sure to delete your local temp file of Roblox wherever that is stored. (You can search for a youtube video if you need more help.) You should also reinstall Roblox.
The issue in the OP (in which non-ascii characters render incorrectly) is fixed now. Recommend starting a new thread if it comes back or if a similar issue arises. : )