Character languages?

Sinbad speaks what's basically Fantasy Arabic, and it's the only living language in his world (and this is a plot point actually, that leads to the reveal the current world is a construct

)

as an adult, he's likely literate in Torran due to the Torran tribe speaking it, and the need for it in dungeons.

and Torran we've seen has an almost Cuniform text so it's probably the equivalent to Sumerian

Joseph English, British-English to be exact with borrowed American slang. As an older man despite his hatred of Japanese, he would've probably learned it for Holly's sake (much as it annoys him to no eend)

past muses

Ky Kiske French is his native language, he's very fluent in English due to a need for a common language in the military, he might know a smattering of german for the same reason. It's however, canon that he does not know Japanese

Snake He's been stated to be fluent in six languages, but we're not told which ones, so I extrapolated from what's said of his military background and Kojima's lunacy: English, Russian, Japanese, French, Arabic, and Farsi