当表扬一个英文小天才时，可以使用以下礼貌用语：

1. "You are incredibly talented in English!"

2. "Your proficiency in English is truly impressive!"

3. "You have a remarkable command of the English language!"

4. "Your English skills are outstanding!"

5. "You are a true English language prodigy!"

6. "I'm amazed by your English abilities!"

7. "You excel in English like no other!"

8. "Your English proficiency is top-notch!"

9. "You have a natural talent for English!"

10. "I'm thoroughly impressed by your English expertise!"。