I’m not sure what you mean. If the translation is incomplete, automatically the English one is used.
Otherwise, translations are handled at crowdin: https://crowdin.com/project/jabref
For example, the version you translate Options to ‘选项’ in chinese, I wonder maybe it can translate Options to ‘选项’ in chinese and add the previous english word like: 选项(Options).
Then we can better get the meanings if the translating is not good, no need to alter the whole language back.
My guess is that getting the right translation is a better way to alter JabRef.
Feel free to suggest better translations at https://crowdin.com/project/jabref .
Cheers.