If by "change language" you just mean change the language under which it is filed whithin Calibre (not
translate the file), yes, and it is easy: just open the metadata window for that file and you will see a "languages" pulldown menu (below, in the central body); choose "English", or whatever.
You can also convert the pdf to ePub. Select the book with the right mouse button, choose "convert books" > "convert individually" and the conversion window will appear; Don't mess with the left hand graphic-based options, leave them as they are; choose "EPUB" in the top right hand corner; while you're there check that the cover and metadata details are correct (text boxes in the right hand)... and click OK (below). That's it.
Don't expect perfect results.
My english is not very good, but I hope that I have made myself understood
.