* Russian: Heifetz was born and raised in Russia, so Russian would have been his native language.
* English: Heifetz spent most of his adult life in the United States, and his recordings and public performances were conducted in English.
* French: French was a common language in high society and the musical world, and Heifetz likely had at least a conversational understanding of it.
* German: German was another major language in music, and he likely had some proficiency in it.
It's possible he had some understanding of other languages depending on his travels and connections. However, it's not known for sure.