Do you start steam with the option LC_ALL=C? (If you do, remove it and you should be fine)
Does it happen too in the chat window? And does it happen only while playing?
Can you give more information on your config and nake90's questions? What locale are you running in?
Closing pending more information.
Nothing extracted yet.
When playing a game, if I open the "Community in Game" and then the browser, I can't use characters with accents like "áéúõóã..." in browser pages.
Using Ubuntu 13.04 64bits with intel graphics card.