Why would you be able to?

Why wouldn’t I be able to distinguish Japanese from English, French, German, etc. ?

No matter how the characters look or act, there is no default language of the made up character.

Dude, this is an anime community. Name 3 anime whose original dub is in English. Or any non-asian language.

They could speak any language or have any voice.

The lipsyncing is done based on the language used in the original dubbing. Which is Japanese 99 times out of 100