Translation:What is the most popular custom of your country?
Why is it the "most" popular in this translation? Should it not be "more" from "mais"?
A better explanation is that "mais" is comparative ("more") whereas "o mais" is superlative ("the most"). It is always superlative with the definite article ("o/a") and always comparative without it.
Comparison: mais=more (or "er" at the end of short adjectives). Superlative: mais = most
Yes. adjectives precede the noun in English, so it must be "the most popular custom". You could say: "Which custom is the most popular in your country" (or, less grammatically correct, but acceptable would be "what custom is....").
O Mais igualmente most. mais igualmente more. Why doesn't the sentence have o mais?
oh, I see. you have the "o", but is placed before the noun!! =) That's the structure."O costume MAIS"..