You can't easily switch between Japanese and English VSQXes. The phonemes between the languages are different. So you have to make sure that each note has the correct phonemes for the correct language.
Please take a look at this manual for Vocaloid Editor (it's for 5, but the phonemes are pretty much the same between the older versions):
https://rsc-net.vocaloid.com/assets/pdf_files/VOCALOID5_Reference_Manual_ENG.pdf
Go to page 65-67 for English, 68-69 for Japanese.
See that, for example, Japanese u sound is "M". But in English, the u sound could be "U" or "u:", there is no "M". Because of the phonemes not being the same between languages, it's impossible to make an English voice bank sing a Japanese VSQx without manually editing the broken notes.
(Note: I am assuming you are using a Japanese VSQx, if not, I think my advice still works, double check that the phonemes aren't messed up. I remember that Freely Tomorrow has some random English in it, so you would have to copy those notes to a different track and have one track for Fukase Japanese and one track for Fukase English.)