Final answer:
Ritchie Valens, an American singer of Mexican descent, famously sang 'La Bamba' in Spanish despite not speaking the language fluently.
Step-by-step explanation:
The Mexican singer who sang La Bamba but couldn't speak Spanish was Ritchie Valens. Valens, whose real name was Richard Steven Valenzuela, was an American singer and guitarist. Despite his Mexican heritage, he grew up in a primarily English-speaking environment and learned Spanish phonetically to record songs such as 'La Bamba.'