Is comparing myself to others when speaking a foreign language in front of my friends—which made them laugh, and which I have repented from—considered backbiting, and must I apologize to them, or is seeking forgiveness and repentance sufficient?

Speaking about people and how they talk with friends and laughing about that is backbiting if what was said about them is true, and slander if it is not true. You have done well in repenting, and the door to repentance is open. You must seek forgiveness from those you backbit, if possible; otherwise, supplicate for them, mention their good qualities, and strive to increase in good deeds that expiate sins.