istrasci’s first translation is good:
(Even) If I send the (mail) in Japanese, will you be able to read it?
(It is a little silly to ask this question in Japanese, because if the answer is “no,” you are not expected to understand the question….)
The で in the sentence is a case marker which signifies place, time, means, cause, and so on, and in this case, it signifies means.
istrasci raised an interesting point: 日本語で (in Japanese) does not state a means of sending a mail. Means of sending a mail include “by air mail,” “by sea mail,” “electronically,” and so on, but “in Japanese” is a means of composing a mail, not a means of sending a composed mail.
However, (手紙を/メールを)送る does not always refer to the mere action of sending an already composed mail. It can collectively refer to the action of composing and sending a mail. In this sense, 日本語で indeed describes a means of this broader action.
As an aside, I note that case marker で etymologically arises from copula. Slightly more precisely, case marker で originates from にて and copula だ originates from にてあり, and these にて are the same thing (the combination of case marker に and particle て). But we usually distinguish case marker で from copula when we talk about the Japanese grammar. As dainichi wrote, this で is no more a connective form of copula than all で particles are.