since when has "gallu" come into this? i remember (waaaaay) back when i was at school, asking someone if the could speak Welsh was "Ydych chi'n siara Cymraeg?"
Cerrtainly the simpler, more concise and more straightforward '(Y)dych chi'n siarad Cymraeg?' is much more common.
But this is an expression that is heard, perhaps more in adult learner circles.
Yes, exactly that.
Although in North Wales the phrase
'Wyt ti'n medru'r Gymraeg' = (lit:-Are you able the Welsh/Can you (the) Welsh) with the meaning 'Do you speak Welsh'
is also common.