Ending the Focus Group Test (FGT) in Korea earlier this February, developer IMC Games teased the multi-language ability for Tree of Savior. A new feature added into the game recently, there is an option to switch between different languages for both text and dialogue.
The first 3 languages are English, Japanese and Korean, as seen in the screenshots here. IMC Games mentioned that testing for Tree of Savior is currently done in Korea, as there might be heavy latency for international users. The studio is examining setting up physical servers in other countries.
There is no mention if the language switch option will be available in the final client, but I guess it will be up to each region’s publisher to decide (it seems IMC Games is prepared to host an international server itself). The language switch option can be used anytime during gameplay.