The word svelte is used in English but it was adopted from the French word (spelt the same) in 1817. The French word came from the Italian svelto, which was an adjective that was made from the verb svellere, which in turn comes from the Latin ex- (“out”) and vellere (“to pluck/stretch”).
As an adjective in English, French and Italian, svelte means “silm/slender/lithe”. It is generally pronounced (using the International Phonetic Alphabet) as /svɛlt/ although some dialects or accents may use a pronunciation variant. It has one syllable in total.
If you want to learn more about the International Phonetic Alphabet, you can find a guide here: International Phonetic Alphabet and Phonemic Alphabets