The reason the left hates him? Let him tell us in his own words,

“In the first place, we should insist that if the immigrant who comes here in good faith becomes an American and assimilates himself to us, he shall be treated on an exact equality with everyone else, for it is an outrage to discriminate against any such man because of creed, or birthplace, or origin. But this is predicated upon the person’s becoming in every facet an American, and nothing but an American…There can be no divided allegiance here. Any man who says he is an American, but something else also, isn’t an American at all. We have room for but one flag, the American flag…We have room for but one language here, and that is the English language….”

Roosevelt believed if you didn't learn the language you should be sent back. Today the corporate world will barely give you a job if you aren't bilingual.