"Let's eat oranges tomorrow."
Translation:Yarın portakal yiyelim.
"Shall I eat oranges tomorrow?" translates to "Yarin portakal yiyeyim mi?" or "... yemeli miyim?" (Though the latter literally means 'should I eat'. You can use different words depending on what you want to say).
"Yarin portakal mi yiyelim?" literally translates to "Shall we eat oranges tomorrow?" emphasis on the oranges. "We" because "-lim" in the verb.
In Turkish, you can emphasize any part of the question by putting the question suffix (mi) after the word you want to emphasize. E.g. These following 3 sentences are translated to English as one but all of them ask different things.
Yarin portakal mi yiyelim? (Asking the object; orange)
Yarin mi portakal yiyelim? (Asking the time; tomorrow)
Yarin portakal yiyelim mi? (Asking the verb; to eat)