Is joking by imitating the language of Indians and saying, "I am a worshipper of cows," considered polytheism (shirk) against Allah, knowing that the speaker did not intend polytheism and feels remorse?

What you have done by uttering this phrase is a serious and grave abomination. Whoever intentionally utters a word of disbelief (kufr) has disbelieved, because he has treated his religion with contempt. He is not excused by jest, error, or forgetfulness, unless he was coerced. Since you have repented, we ask Allah to accept your repentance. You must be cautious about this in the remainder of your life.