Number of SQL Server languages As per MSDN


  There are currently ten SQL Server languages. The report server
  language for a given report server instance will always be one of the
  ten available languages. Support for all ten of the SQL Server
  languages is provided in a single Reporting Services installation.
  There are no language packs to install.


However, as per the page for sys.syslanguages there are many more listed

How should these 2 pages be interpreted together? Once says there are 10 SQL Server languages, and Report Server supports those 10. The other says there are many more, so are these supported by Report Server or not?
You get 11 languages (well, 10 if you count us and british english as the same) from SELECT DISTINCT language_id FROM sys.messages
and 33 from SELECT DISTINCT lcid FROM sys.syslanguages.

If not listed in sys.messages, messages for the default language are used.

As for SSRS:


  If you install Reporting Services in a default configuration, Setup
  determines the report server language when it creates the report
  server database. The language resource that Setup uses is the one that
  mostly closely matches the language of the operating system: If an
  exact match between the operating system language and a SQL Server
  language is found, Setup will use the exact match. For example, if the
  operating system language is U.S. English, Setup will use the SQL
  Server language resource for U.S. English as the report server
  language. If an exact match cannot be found, a close match will be
  used instead. For example, suppose the operating system is U.K.
  English. Because U.K English and U.S. English are in the same language
  family, Setup chooses the SQL Server language resource that most
  closely matches U.K English (in this case U.S. English). If there is
  no close match available, the report server language will be U.S.
  English.


https://msdn.microsoft.com/en-us/library/ms156493%28v=sql.105%29.aspx