Greek υπότιτλοι with the accents where they belong.

Greek has two lowercase sigmas: σ in the body of a word and ς at the end. They are not interchangeable, and they are different characters in the file. The practical risk in captioning is any operation that splits or rejoins words — a search and replace, a hyphenation experiment, a tool that truncates a cue — because it can leave a final sigma in the middle of a word, which reads as an obvious error.

The accent is the other one. Monotonic Greek puts a tonos on the stressed syllable of every polysyllabic word, and it distinguishes words that are otherwise identical, such as πότε and ποτέ. Greek convention drops the accent when a word is set entirely in capitals, so styling your captions as all-caps in an editor should remove the tonos rather than keep it — while the diaeresis on ϊ and ϋ stays in both cases.

• Greek quotation marks are the guillemets « », and a dash opens a line of dialogue in printed convention.
• The question mark is a semicolon-shaped ;, which is correct in Greek and not a typo to be repaired.
• Words are long enough that Greek lines hold fewer words than English ones at the same width.
• Keep the file UTF-8, since legacy Greek code pages lose the accented vowels on the way to a modern player.

Neither downloaded format announces who is speaking: SRT and VTT are cue text and timecodes. The separation between voices is visible in the transcript view, and is stored in the JSON export from the same job.