"Dewi Lingo is the manager."
Translation:Dewi Lingo yw'r rheolwr.
As EllisV says, y rheolwr and Dewi are one and the same, so to say that we must use yw/ydy, not mae. Which one comes at the front if the sentence depends on which one we want to emphasise:
- Dewi yw/ydy'r rheolwr - Dewi is the manager (Not Siân or Siôn, say)
- Y rheolwr yw/ydy Dewi - Dewi is the manager (He isn't the cook or the finance director, say)
If anything "Mae Dewi Lingo yn rheolwr", but that means "Dewi Lingo is a manager". There you have two forms of "Bod" i.e "Mae" and"Yw". We would use the emphatic here because we are equatiting "Dewi Lingo" to "Rheolwr".