Romantic.

From Japanese 浪漫 (ろ​ーまん [ɾo̞ːmã̠ɴ]), which is a case of phono-semantic matching (Wikipedia) where a foreign word (roman) is replaced with native words with similar sound and meaning. Individually, 浪 means "wave" and 漫 means "overflowing; free" so the whole expression evokes a quite poetic image of crests and troughs of waves crashing on shores. This word is purported to have been coined by Natsume Sōseki, often considered the greatest writer in modern Japanese history.

The pronunciations of lãng mạn and roman bear almost no resemblance at first glance because Vietnamese and Japanese both borrowed heavily from Classical Middle Chinese of about 1500 years ago and since then the pronunciation of both languages have diverged significantly.