"sounds" is correct. As for English or English language it's the same. You can use English language to make emphasis that it was an english language class

Neither are really correct. "... I learned which one sound in the English language most important ." the meaning is unlcear whether sound means pronunciation or "seems like" , and important is wrong for the following reason: Today I learned which one(word) sounds more important in English. ---is in correct because a word or pronunciation should sound more formal or less formal, or is used more often or less often , not described as important. "the " is necessary before "English language" but it is perfectly acceptable to say "in English"