"Cò às a tha thu Ealasaid?"
Translation:Where are you from, Elizabeth?
Question about your response: I referred to you as thu with leat but you referred to me as sibh with ur. Is there any usual practice with people you don't know online? I ask because in Welsh I think I'd tend to use ti (i.e. Gaelic thu) rather than chi (= sibh) here unless something in particular pointed out that someone was older than me. (Maybe it's because I assume everyone's a similar age or younger than me on Duolingo forums.)
Yes - or 'who(m)'. When followed by a preposition, it it best to translate cò with whatever interrogative sounds best in English, rather than trying to remember specific rules as to which you can or should use. The point is that it is general purpose in this situation, with no implication of animate/inanimate etc.
Of course it meant 'who' 'originally' as the o shows it is the same word, in the same way that cuin means 'when'.