The language is normally determined by users the browser settings. settings of a user. This language can be overriden overridden by changings changing the locale on a of the session. See the public API reference: [ISession#setContentLocale(java.util.Locale)][1]
to modifiy modify the task e-mail content language, there is another API: [IUser#setEMailLanguage(java.util.Locale)][2]
[1]: http://www.xpertivy.ch/releases/Xpert.ivy/5.0.6/documents/publicApi/ch/ivyteam/ivy/security/ISession.html#setContentLocale%28java.util.Locale%29
[2]: http://www.xpertivy.ch/releases/Xpert.ivy/5.0.6/documents/publicApi/ch/ivyteam/ivy/security/IUser.html#setEMailLanguage%28java.util.Locale%29
