If your computer is set to a specific language, and that language is Spanish, Japanese, or Swedish, Stata will recognize this and automatically use that language. You can easily change the language by selecting Edit > Preferences > User-interface language for Windows and Unix or Stata 15 > Preferences > User-interface language... for Mac.
StataCorp gratefully acknowledges the efforts of LightStone Corporation, Stata's official distributor in Japan, for the Japanese translation. We also gratefully acknowledge the efforts of Metrika, Stata's official distributor in the Nordic countries and Baltic states, for the Swedish translation.
See New in Stata 15 for more about what was added in Stata 15.