Around December 2014 there has been an update to the CSOM model for on-premises SharePoint 2013. The update unfortunately included a bug in AddSupportedUILanguage() function: [
Update on 20th of Dec - Adding additional language to web has unfortunately a bug and it does not work unless site has already one or more additional languages set, which means that this API does not really usable until this bug is fixed. This will be fixed for future releases and to redistributable version of the CSOM package.
As stated above, the bug was supposed to be fixed in future releases. I am currently using a release for SPO from 9April 2015, and the bug still persists. Any ideas as to where it has been fixed (maybe I have some wrong release) or when it is going to be fixed?