Q: Are lawyers or rabbinic advocates acceptable in beis din?
A: It is preferable that the Dayanim in beis din hear directly from the litigants, whereby they can get a clearer picture of the truth, similar to the Biblical obligation for witnesses to testify orally. For this reason beis din should also understand the language of the litigant, and if possible not rely on a translator’s interpretation (C.M. 13:3; Sma 12, 17:6, 14; Kovetz Haposkim ad loc.).
Nonetheless, the common practice nowadays is to allow for a legal advocate (to’en) to help a litigant to better voice his position. The to’en may also add halachic arguments in his client’s favor, after the litigant has presented his arguments. It is also common practice to allow a legal advocate to present his client’s arguments if the litigant cannot appear personally in beis din (see C.M. ibid., Aruch Hashulchan 124:2).