"Sut un yw Dewi Lingo?"
Translation:What is Dewi Lingo like?
For "day", there are certain contexts when you'd used dydd and when you'd use diwrnod. When describing, e.g. using an adjective, the word for "day" is usually diwrnod: Sut ddiwrnod gest ti? (What kind of a day did you have?) Ces i ddiwrnod neis/prysur/diflas (I had a nice/busy/boring day).
The same applies for "night". With a description you'd used noson, not nos: Sut noson gest ti? (What kind of a day did you have?) Ces i noson hyfryd/dda/dawel (I had a lovely/good/quiet night).
Is this literally "How a one is Dewi Lingo", meaning "what type of person is he"?
You can't use beth before a noun or number like this. You can say:
Beth yw Dewi Lingo? "What is Dewi Lingo?"
Beth un yw Dewi Lingo? (Doesn't make any sense.)
Question words you can use before a noun or number include sut "what kind of":
Sut un yw Dewi Lingo? "What kind of one is Dewi Lingo? / What is Dewi Lingo like?"
or pa "which":
Pa un yw Dewi Lingo? "Which one is Dewi Lingo? / What one is Dewi Lingo?"