"Where is New Zealand?"

Translation: Ble mae Seland Newydd?

[quarter_moon]

Shouldn't it be "ydy"/"yw" here instead of "mae" as it is a question?

[ibisc]

No. 'Adverbial' question words such as Ble/Lle, Pam, Sut, Pryd? (where, why, how, when?) are always followed by mae..., bydd..., roedd..., etc rather than by ydy/yw..., fydd..., oedd..., etc. (Althought the r- in forms such as rwyt, roedd, etc is sometimes dropped in speech.)

[quarter_moon]

Thank you very much for the clarification!

In lesson "Extend1" e.g. there is "Beth yw ei farn?" Maybe "beth" is from another category of interrogative pronouns?

[ibisc]

Yes, beth and pwy are usually asking who or what did something or had something done to them.