"Who is the technician?"
Translation:Pwy ydy'r technegydd?
No. If you asking who in particular has a particular job or function or name, then it must be ydy. If you are asking who in general is a whatever, then it is sy:
- *Pwy ydy'r technegydd? - Who is the technician?
*Pwy sy'n dechnegydd? - Who is a technician?
Pwy ydy Dewi Lingo? - Who is Dewi Lingo?
- Pwy sy'n dysgu Cymraeg yma? Who is learning/teaching Welsh here?