The Halacha is that we cover the Challa / Cake while we make Kiddush in order not to embarass it (Talmud Yerushalmi as quoted by the Tur, Orach Chaim 271) as we are not making Hamotzi / Mezonos first. Is a clear covering adequate (plastic / cellophane) or does it have to be non see through?
I remember at an "in Shabbos" at Yeshivah Gedolah Ateres Mordechai in Detroit there being Hallah on the tables Friday night in just their see-through bags. Some boys took napkins and started covering them. Rav Leib Bakst ZT"L, the Rosh HaYeshivah, did not say anything until they reached his table. When they reached his table he said "What?! You're worried about "embarrasing" the Challos?! Don't worry! They are not embarrased in plastic bags!"
He held that even lechatehilah they are good enough.
(I muttered to the guys who were covering them "They can't be 'embarrased' anymore because they suffocated in those plastic bags!)
In the back of the Minchat Shabbat (Shiyurei ha-Minchah 77:8), a commentary on the Kitzur Shulchan Aruch, (mentioned in this answer) it says that an additional reason we cover the Challot is because we don't want to embarrass them by skipping over making a blessing on them in order to bless the wine first.
Based on this, he says that one should make sure that the cover being used does not have holes that would enable one to see the bread.
He sources this to "Achronim".
If so, the same logic should apply to see-through bags.
L'chatchilah, no, since the challos can be seen. If no other cover is available, such covers may be used.(See Minchas Shabbos (Shiyurei ha-Minchah 77:8), Tikunim u'Milluim 47, note 116 and Bris Olam, Kiddush 34.)