MechMachineMan is right. The parameter way is kinda uncanny, as you'll need these parameters in every part/assembly you want to use it in.
If the form is somehow final, you're not planing changing it and the languages aren't many you can use this instead:
1) Finish your Form with one languange and save it as GLOBAL FORM.
2) Go to your DesignData folder and there to "iLogic\UI", standartly "C:\Users\Public\Documents\Autodesk\Inventor 2016\Design Data\iLogic\UI".
3) Copy your form ("FormNameEN.xml") to something like "FormNameDE.xml"
4) Change the language in the new file.
5) Create a global rule that will open the form based on the Inventor's language settings.
Consider using "Accept as Solution" / "Kudos" if you find this helpful.
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Regards,
Mike
"Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live." - John F. Woods