Student or Learner
Could you tell me the difference?
A letter written in English.
A letter written by English.
A letter written with English.
"In English" means in the English language.... "by English" could mean by a man named English (rare); "with English" could mean with a man named English, but is unlikely, or, it could mean a letter written in another language with some English, but is phrased badly if that is the intended meaning. So, you may regard the first as being the only good answer, as 2006 says.