Sebastian explained it in his tutorials, but they are in german.
First you go to the "game info" tab. There you can add languages. (see image)
When writing text for your characters you can then choose the language via the little flag. (image) Or you can translate the texts via the "texts"tab.
And you need a button for the player to change the language. Create a button for each language in a menu or interface. Put an action there. Like: action "leftclick" - action part "change language to" [german] (image)
If you want to change some objects when changing the language (like menu buttons) you can do this by creating a condition or value that you change when switching the language.