Players who already own the game will get the new options via today’s localization patch. Language settings can be changed by navigating to game properties in the Steam library, and selecting a language there before launching the game. The language options cannot be changed while the game is running, so the selection must be made before the game launches, or by restarting the client. Localization is currently only for UI and menu elements only.
Other languages are also already in the pipeline, and the team are planning to add French, Spanish, Portuguese, Italian, German, Polish, and Turkish in a later patch as well.