I think the people who first spoke Vietnamese didn’t want outsiders to learn. They came up with 6 tones, which they denote by putting various marks on top of vowels. It’s also a sing-song language. No singing = no Vietnamese.
English: Tư commits suicide slowly.
Vietnamese: Tư Tự Tử Từ Từ
English: The size of the warehouse is small; hence drying is difficult and miserable.
Vietnamese: Khổ nhà kho nhỏ vì thế độ khô khó và khổ
The language has many traps. Thieu means pepper, Dieu means cashew, and Thieu Dieu means disaster. Similarly, some word with a cap on the vowel could mean ‘God is great’, the same word with a question mark on the vowel could mean ‘you fucking bitch!’