Messi played football in Spain since childhood, so it is better to write in Spanish. If not, English should be fine. Please write the address to the Barcelona Club, using only the general letter format. You can express your love for his technology in your letter. You can say something about his game, which impressed you deeply. Finally, I can also wish Barcelona the championship this season. Time is not important. Just express your sincere feelings.

In addition, a big club like Barcelona will have a special public relations department to handle letters from fans, and it depends on luck to get a reply. I think you can attach photos or something to the letter. If he is in a good mood, he may sign it back to you.