Soko comes from Arabic سُوق (sūq, "market, souq, bazaar, street of shops"), from Aramaic שוקא or ܫܘܩܐ (šūqā’, “street, market”), from Akkadian sūqu (“street”), from sāqu (“narrow”). Compare Hebrew שׁוּק (shuk, "market in the broad sense; a street market"). Spanish zoco ("market") and English souq ("street market") come from the same Arabic root.
I think that "We would like big markets." can be accepted. I understand that omba means request/ask for, it has cognates in the many Bantu languages I encounter, but the translation ''would like'' seems more suitable in this case. It makes more sense to an English ear. Translation is more than just word-for-word.
Yes, it is a matter of politeness. It is similar to saying "I would like some milk" vs "I want milk." They might mean basically the same thing, yet the tone is very different. As a parent of a 5 year old, this is something that we discuss in our house quite often.