Documentation
Language definition
While building a lesson, you define its language in the presentation's preferences/settings. If you are going to use the Text To Speech module, this is the language in which all speech texts are read. This is also the language in which, by default, all modules are read, however, you can change the language for each module by defining it in the module's Lang attribute.
It is also possible to define inside a module a fragment of text that should be read in a different language. You can define as many such fragments as needed, and each one can be read in a different language.
You can use the \alt{text|text}[lang lang_attribute] syntax, and "text" will be read in the language defined by the lang_attribute.
In the documentation of the Text To Speech module, in the section Languages, there is a script that checks what languages are available to you in your browser. You can add more languages by adding them to your operating system.