I heard that Rashi uses a Russian word where he deals with the building of the Beis Hamikdash by King Shlomo. I also heard that he uses a Polish word.
Does anyone know where it is?
In his commentary to I Kings 6:7:
Although it seems quite likely that this is a later interpolation; it doesn't appear in early prints of Rashi.
In several places, though, Rashi refers to לשון כנען, which was a popular term at the time for the Slavic languages (based on the equation of "Slav" with "slave" and the association of the latter with Canaan). These include Deut. 3:9 (שניר הוא שלג... ובלשון כנען), Shabbos 20b in Hagahos HaBach (פסולתא דזיפתא... עיטרן שקורין דוהינו בלשון כנען), Avodah Zarah 28b (חיפושתא... ובלשון כנען קרוקי"ם) and 51b (דסחיפא לה משכילתא ארישיה... כלי ארוך ובלשון כנען אקדון). Some of these might indeed be Polish, although I don't know.