The Shulchan Aruch in O"Ch 285 mentions using Rashi in lieu of, or perhaps in addition to, Targum Onkelos.
What is the origin of this practice?
- - - - -
R Yaakov Jaffe
* * * * *
This idea is cited in the Smag, who says "danti lifnei rabosai" who agreed to his position (as cited in Beis Yoseif 285).
Interestingly, we know that Rabbi Yehudah Sir Leon was the Rebbe of the Smag, and we also known that Rabbi Yehudah Sir Leon is the author of Tosfos to Brachos - so the Smag must be understood in light of Tosfos in Brachos (8a), where a non-aramaic translation may be used. Tosfos there take the first step away from the Divinely-inspired Targum Unkelos as the only option for "echad targum," allowing for whichever translation works best for the reader, and so the natural progression taken by his Talmid shortly thereafter is that Rashi's translation and interpretation is no worse than a French translation.
Thus, perhaps it is appropriate to say "The Smag - working off of a statement of his Rebbe in Tosfos"