This is a pretty advanced question, but I understand other games have done it by using a language file containing all text strings like .gxt
In GDevelop this will probably involve the use of a JSON file (never did it though) and accessing it every time a text is displayed, to change the label to the correct language.
For example, a text might show “Options” by default.
In the beginning of the scene, there would be an event to access the JSON file and change what the text displays, according to the language global variable.