Learn Turkish: En arka sıraya oturalım. - Let's sit in the very back row.

The root verb is oturmak (to sit). To express let's do something (the optative mood), we add the suffix -alım or -elim. Since the last vowel in the stem otur is a back vowel (u), we follow 2-way vowel harmony and use the back-vowel version, making oturalım.

Why does the verb oturmak take the dative case in sıraya instead of the locative sırada?

In Turkish, when you are talking about the action of moving to sit down onto or into a place, oturmak takes the dative case (direction towards). Sıra (row) + y (buffer) + a (dative) = sıraya. If you were describing the state of already being seated there, you would use the locative case instead, like in sırada oturuyoruz (we are sitting in the row).

I learned that en means the most for superlatives. How does it mean the very here?

You are right that en is usually the superlative marker (like en güzel for the most beautiful). However, when used with spatial position words like arka (back), ön (front), or alt (bottom), it translates better as the very or the furthest. So en arka means the very back.

How would I change this sentence to ask Shall we sit in the very back row?