To edit the assigned language list of the active document choose "Edit Document Languages" from the language settings button in the modeling ribbon tab.
The Edit Diagram Language Dialog provides the following options:
To Add languages hit the Add Button. Then select one or more languages from the list of available languages and click OK.
Adding new languages to the diagram language collection
To remove languages select one or more languages from the list and hit the Remove Button. There must be at least one language remaining to be able to confirm the setting with OK.
The Default Language is the language, from which the visualization is taken if a shape is not translated in the Current Language.
In this example English is the Default Language. Spanish is the Current Language. Because the term has not been translated into Spanish, the English one is shown, but enclosed in ">>" resp. "<<"