Set the visitor UI language automatically

Ok, final answer, I believe, you need the following to set the user profile lanugage:

<#assign userLang = "fr" /> <#-- or whatever you want it to be -->
<#assign resp = restadmin("/users/id/${user.id?c}/settings/name/profile.language/set?value=" + userLang?url)/>

 Unfortunately, the documentation for setting the profile.language (or really, any setting) does not include the critical last part of the correct API call, 

https://community.yoursite.com/restapi/vc/users/id/[user_id]/settings/name/[setting_name]/set?value=[language_id]?url  

or, for example, 

https://community.khoros.com/restapi/vc/users/id/1234/settings/name/profile.language/set?value=fr?url 

where?url indicates that it should be url encoded.

Ok, hope that all answers your question enough for you to get a working solution. If not, mdfw just tag me here and I'll try to help some more 

- Ian