I do have a sample of changing a forms properties based on a language selected in a drop down list. But this one stores all the language values in an XML file and binds the captions, error messages, tooltips, display patterns, etc using the <setProperty> command. For a caption and some error messages you can set this in the Designer UI but for others you need to add them to the XML Source of the form directly. It is easy enough once you see the pattern.
After all the <setProperty> commands are added the only code you need is an "xfa.form.remerge();" in the exit event of the language selection drop down list.