Languages

From Player's Handbook, page 123.

Your race indicates the languages your character can speak by default, and your background might give you access to one or more additional languages of your choice. Note these languages on your character sheet.

With your DM's permission, you can instead choose a language from the Exotic Languages table or a secret language, such as thieves' cant or the tongue of druids.

Some of these languages are actually families of languages with many dialects. For example, the Primordial language includes the Auran, Aquan, Ignan, and Terran dialects, one for each of the four elemental planes. Creatures that speak different dialects of the same language can communicate with one another.

Languages and Dialects
(Dungeon Master's Guide, pages 20-21)

When fleshing out your world, you can create new languages and dialects to reflect its unique geography and history. You can replace the default languages presented in this page with new ones, or split languages up into several different dialects.

In some worlds, regional differences might be much more important than racial ones. Perhaps all the dwarves, elves, and humans who live in one kingdom speak a common language, which is completely different from that spoken in the neighboring kingdom. This might make communication (and diplomacy) between two kingdoms significantly more difficult.

Widely used languages might have ancient versions, or there might be completely different ancient tongues that adventurers find written in tombs and ruins. Such languages can add an element of mystery to inscriptions and tomes that characters encounter.

You might invent additional secret languages, besides Druidic and Thieves' Cant, that allow members of certain organizations or political affiliations to communicate. You could even decide that each alignment has its own language, which might be more of an argot used primarily to discuss philosophical concepts.

In a region where one race has subjugated another, the language of the conquerors can become a mark of social status. Similarly, reading and writing might be restricted by law to the upper classes of a society.