Sorry, I didn't knew.
You can't modify in any way pressed discs.
For what I know the only solution is doing a complete backup modifying settings with a tool like IFO Edit, so you can create a backup that start with the right language.
You can find guides on ifoedit here
But if you must anyway do a backup and you are interested only in main movie without menus or other contents, then dvd shrink is the tool for you.