When someone starts speaking to me in English I often want to effectively say "Japanese is fine".
I usually blurt out 日本語でもいいです which I thought would mean "in Japanese is also ok" but someone once told me I should say 日本語もいいです "Japanese is also ok".
I interpret the first 日本語で as "in Japanese". A longer sentence might be
you are/I am speaking in English but speaking in Japanese is also ok
which seems like it can be shortened to
you are/I am speaking in English but in Japaneee is also ok
I am/you are speaking in English but Japanese is also ok
dropping the で just seems to remove the "in" as in the "way" of speaking.
is でも ok here or not?