This is a quick slap together attempt, don't kill me if it's bad, I am a super amateur at translating, but I had to do it now or I wouldn't be able to sleep.
yan yan a,yan
what is the deal*, yadon
yan yan a,yan
what is the deal, yadon
Waking with the rising sun in a dreamy daze, but sleeping until noon. Yadon.
Pink skin that seems really oily, but they're just born that way. Yadon.
A giant mouth that's open all day, just like a shopping mall. Yadon.
Their tail has a sweet taste, sometimes kids have it for an afternoon treat. Yadon.
They move with a crawl so their pictures never blur, so I guess that's something to be proud of? Yadon.
yan yan a,yan
what is the deal, yadon
yan yan a,yan
what is the deal, yadon
Cut off their tail, there's no worries, another will grow instead. Yadon
Who was the one who named this the "Dopey Pokemon"? Yadon.
Start with a "ya", end with a "n", three letter name and you get: Yakan.
Uses tail for fishing, doesn't care if a bite, doesn't care if it gets away. Yadon.
If you play "shiritori", then Yadon's name I cannot recommend.** Yadon.
yan yan a,yan
what is the deal, yadon
yan yan a,yan
what is the deal, yadon
With a single yawn, they can call a rain storm. A legendary rain chant. Yadon.
But now there isn't a cloud to be seen, what happened to the legend? Yadon.
If a Shelder bites the tail you get a Yadoran (SlowBro), even the evolution's dopey. Yadon.
Yadon are everywhere in the wide world, and now there's even one behind you. Yadon.
Yamakake, moon gazing, tempura, gomoku, southern curry. Udon. ***
yan yan a,yan
what is the deal, Udon
yan yan a,yan
what is the deal, yadon
A giant pice of fruit could fall on their head but they won't feel a thing. Yadon.
Their ears roll over and make a spiral shape...what is up with that? Yadon.
Suddenly they say [Yan], is it because falling fruit from before hurts now? Yadon.
Maybe Yadoran, even Yadoking (SlowKing), or just staying the same, Yadon.
Everyone bites a tail, join together to make a ring, and they seem so happy for it. Yadon.
yan yan a,yan
yan yan a,yan
yan yan a,yan
Yadon.
*"Donaiyanen" from what I've seen and did 10 minute Google/Dictionary research on isn't a normal word and seems to be a kind of slur between words and accents that comes up when a person is exasperated with someone else. I have the least amount of confidence in this translation and I would greatly appreciate if someone smarter could correct me. My impression is that since the song is about how strange Slowpoke is, the singer straight up asks "What are you?"
**"Shiritori" is a word game where you pick a word and then think of another word that starts with the same letter as was the last one of the first word. For example "Shee
p" and then "
pig". However there's a rule that you cannot use a word that ends with "N (ん

" or you lose because there are almost no Japanese words that starts with "ん" which is why "yado
n" is not recommended. "Choose Slowpoke and you're gonna lose!" is the joke I believe.
***Just a bunch of food. Not super sure what the point is or if accurate. Again, not confident in my barely amateur translations at all.