The language of the text in menus and dialogue boxes is controlled by your System Preferences. To fix this:
Open your System Preferences and go to the Language and Region preference.
Under Preferred languages, you'll see a list of the interface languages you can use, and other preferences. Drag English to the top of the language list and you'll be able to enjoy English languages Menus in Mellel.
Note that the change is only applied to applications when you re-run them.