Stream Buddy provides three different languages at launch, with more languages coming as we move from Early Access to Full Release. The current languages provided are English, French, and Russian. To set the language, open the option menu by pressing O on your keyboard. At the bottom left of the option menu there is a drop down combo box which will allow you to select the language of your choice.
By default, the first time you open Stream Buddy it should automatically adjust to your system language if your operating system locale is set to English, French, or Russian. However, if it does not, you can manually set the language based on the instructions above.