"My mother eats eggs for breakfast."
Translation:Moja mama je jajka na śniadanie.
You cannot translate Polish prepositions word-by-word, they work just for given contexts. When taking about what is eaten for any meal, in Polish we always use "na".
And BTW I would say, that the English preposition for is "extremely overused" compared do Polish, because it translates to a bunch of other prepositions in Polish, depending on context, and even within given context, their usage varies depending on verb/noun/pronoun involved. See:
In "Polish Grammar in a Nutshell" by Oscar E. Swan, there is a chapter about Polish Prepositions with a special paragraph about translations of "for". Here is also a good article about Polish Prepositions: "Everything You Need to Know about Polish Prepositions" by Adam Łukasiak.