According to the documentation, Mendix states the following:
For the language of the document, the document generation service uses the language of the user that is passed in the
Generate as user property. If no language is configured for the user that is passed in the
Generate as user property, the default language as set in the App Settings in Studio Pro is used.
So I guess that either the text used in the PDF hasn't been translated yet, or you haven't set the right language for the user you're trying to accomplish this with.
Hope this helps.