1) PREPARE THE APPLICATION
To translate an application, all translatable strings must be marked using one of the following functions: 
- this.tr(): translate a message
- this.trn(): translate a message that supports a plural form
- this.trc(): translate a message and providing a comment
- this.marktr(): mark a string for translation, but do not perform any translation
You can use these methods right away for your own classes if they are derived from qx.ui.core.Widget or qx.application.AbstractGui. 
If thats not the case you have to include the mixin qx.locale.MTranslation manually:
qx.Class.define("custom.MyClass",
{
  extend : qx.core.Object,
  include : [qx.locale.MTranslation],
  ...
});

Example Change original code like this:
var button = new qx.ui.form.Button("Hello World");
to:
var button = new qx.ui.form.Button(this.tr("Hello World"));

2) CHECK YOUR CONFIG 
After the source code has been prepared, the desired languages of the application may be specified in config.json, 
in the LOCALES macro within the global let section, for example
This would add a German and a French translation to the project. For a more exhaustive list of available locales see here.

"let" : {
    // ...
    "LOCALES"       : ["de", "fr"]
}

3) GENERATE/UPDATE TRANSLATION FILES
This is done automatically during the maven build process.