I would say there is no difference between "am Tag" and "am Tage", but that might depend on the context you have in mind. If you think about the period "during the day" then maybe it is even better to use "tagsüber".
"am Tag X" = "am Tage X" ("on day X")
(you cannot use "tagsüber" here, it's a different context)
am Tage schlafen die Feldermäuse = tagsüber schlafen die Fledermäuse
(I think it is possible to use "am Tag" here as well, but it sounds a bit strange)
Regarding your second sentence: You can certainly *not* replace "heißt" (a verb) bei "wie" (an interrogative pronoun).... But you can ask "Wie kann ich xxx auf Deutsch sagen?"