What are the lyrics to 'Happy Birthday to You' when translated from English to Japanese?

Otanjou-bi Omedetou Gozaimasu!

it means Happy birthday to you, so just repeat it using the same tune as Happy birthday in English

Some Japanese also use the Happy birthday song in English but with Japanese accent which is like this:

Happii baasudei tu yu

Happii baasudei tu yu

Happii baasudei tu (name)

Happii baasudei tu yu