Translation:It's Mr Lingo who is wearing white clothes.
This is an emphatic sentence with the usual word order changed. In English we would tend to say '(It is) Mr Lingo (who) is wearing white clothes, although we could say 'Mr Lingo IS wearing white clothes'
'Mr Lingo wears white clothes' is a more normal statement and is translated as 'Mae Mr Lingo yn gwisgo dillad gwyn'
The Welsh could also be translated as 'Mr Lingo is wearing white clothes' but without the emphasis.
'Mr Lingo is wearing white clothes' is not accepted, but I think it should be. As you say, tone of voice is used for emphasis.
If I heard someone say "It is (so-and-so) who is ..." , I would think that person trying to mimic a stereotypical (to Americans) Irish speech pattern. It doesn't sound like standard (American) English.
(PS - I see I commented on this 5 months ago. I must be getting curmedgeonly; I dislike the official answer more now than I did then.)
I wrote, "mister lingo sy'n gwisgo dillad gwin" and got wrong. It seems I should have written Mr. I don't understand. The voice certainly SAID 'Misteer Lingo' not Mr.
Why is "It is Mr Lingo who is wearing white clothes" marked as wrong? Is there a reason why it has to be Simple Present and not Present Progressive? So far, it seemed to me that without specific markers like "ar hyn o bryd" or "bob dydd", both tenses were acceptable as translations...