You can customize the HTML/PDF act language to every Object. It is really simple.
Simply go to your Object list.
Click New Object > Settings button:
Look for a field called Locale:
Make sure it has a green check mark next to it.
Open the Object's Editing screen and add a new value in the field Locale
Remember, the language will only apply to the new PDF/HTML acts, you will have to regenerate the older ones to change its language.
How to regenerate act?