Reply Assistant automatically detects the language of current email and uses the salutation etc. in the correct language.
Unfortunately it is not always possible to detect the language automatically.
Particularly in the following cases the detection may fail:
If the language was not correctly detected clock on the language button in the Reply Assistant area (left or right side) in the email window and select the correct language.
If you are using the "Inline Reply" feature of Outlook, you will find the language button in the Ribbon of Outlook in the Group "Reply Assistant" (for details see Outlook Inline Reply (Outlook 2013)).