Formal:
* "I'm impressed to hear you speak four languages! Which ones are they?"
* "It's amazing that you are fluent in four languages. What a valuable skill!"
* "Your multilingualism is truly remarkable. Which languages do you speak?"
Informal:
* "Wow, you speak four languages? That's so cool!"
* "You must be able to communicate with so many people! Which languages do you know?"
* "You're a polyglot! That's awesome."
In a conversation:
* If you notice them switching languages seamlessly, you could say, "You're so fluent in all these languages! It's incredible."
* You could ask them, "How did you learn to speak so many languages?"
* If they mention they are learning a new language, you could say, "That's great! I'm sure you'll be speaking it fluently soon."
Important Note:
* It's always good to avoid any phrases that might sound condescending or belittling.
* Focus on your genuine admiration and curiosity about their language skills.
Remember, the best way to say it is to be sincere and respectful.