Translation:He would be delighted to play for Wales.
I'd say chwarae i is definitely more commonly heard these days. Google gives nearly 14,000 hits for "chwarae i Gymru" as opposed to 5,500 for "chwarae dros Gymru".
They have different meanings.
'dros' is used in the context of representing and is found in other situations.
eg the member of parliament for Swansea = Yr aelod seneddol dros Abertawe
So playing for Wales in a representative role = chwarae dros Gymru
On the other hand playing for a specific team:-
eg playing for the Wales Rugby team = chwarae i dîm Rygbi Cymru, which then is often contracted to 'chwarae i Gymru'
I understand what you're saying, yeah. chwarae dros and chwarae i aren't entirely synonymous, but when it comes to "play for" in English, chwarae i is much closer. A more accurate translation for chwarae dros would be "play on behalf of/representing" because it sounds slightly stilted as does the Welsh. I've heard more than one native speaker comment on how chwarae dros is taught in classes although it's not nearly as common or current as the courses would make out. It has a slightly old-fashioned feel, but nice nevertheless.