It makes no real sense that Humans in the Ostschild setting would speak Dwarf, or Goblin or Elfin or any of a dozen or more other fantasy languages common to D&D. This is a more historically based setting, so I thought I'd toss in some historical languages. All PCs are going to speak the common language of Humans in Ostschild, which is Frankish, by a nose over Wendish.
Dwarfs will speak Dwarf, which is said to be similar to the tongue of the Northmen. Elfs will speak Elfin, the language of the courts of Faerie. Halflings have no special tongue of their own, but speak the tongues of men in their vicinity.
The Languages of Men
Frankish not only hangs on in Ostschild as a prestige language, even as it evolves into French to the west; but it prospers, replacing the Wendish tongue in the region. Frankish is a Germanic language, with a lot of Latin loan words, not unlike modern English in that regard.
Wendish is a Slavic language, the ancestor of modern Czech and Slovak, it is the language of the population that was conquered by Charlemagne.
To the North – German, Norse.
To the East – Polish, Russian, Cuman.
To the South – Italian, Romanian, Serbian, Magyar.
To the West – German, Wendish.
Further afield – Greek, English, Breton, Bulgarian, Irish, Welsh, French, Spanish, Occitan, Turkish, Arabic, Farsi, Dutch, Finnish, Basque. All of these languages have lots of dialects and regional variants, D&D isn't a language simulator though, and adding that kind of complexity seems kind of pointless, so I have made some arbitrary decisions about languages here. Hell, maybe this is already too complex.
Latin,Greek, Hebrew, Aramaic, Arabic. The languages of the old and new testaments, plus the Koran. There are other holy languages in the world than these ones obviously, Sanskrit comes to mind, but these are the languages that are likely to be seen in the Ostschild setting.
Latin, Greek, Hebrew, Aramaic, Arabic, Sumerian, Demotic, Coptic, Etruscan, Minoan, pretty much any ancient language will work. I am adding this category, because I am always tinkering with the game, and my current thinking is that Magic-Users keep their Grimoires (Spell Books) written in some ancient tongue rather than in some special esoteric “magical” language.
Dwarf and Elfin are the two key languages here. I am unsure whether or not the campaign really needs any others. My current thinking is that Goblins and other Fey creatures probably speak some lower class dialect of the Elfin tongue. Dwarfs speak their own elder Nordic tongue. I honestly don't see any need for a “Draconic” language, or really any other fantasy languages. In folklore the odd creatures all speak the tongues of men anyway.