* Serbian: His native language.
* Croatian: A language closely related to Serbian, spoken in the region where Tesla grew up.
* Czech: While not entirely fluent, he had a good understanding of Czech due to its Slavic roots.
* German: He learned German during his studies in Austria and used it extensively for his scientific work.
* English: He became proficient in English after moving to the United States, and used it throughout his career.
There are also accounts of Tesla speaking French and Italian, although the extent of his fluency in these languages is unclear.
So, while we can't be certain, it's safe to say that Nikola Tesla was a polyglot who was comfortable communicating in at least five languages, and likely had some understanding of several others.