In LibreOffice 4.3 the user will be able to specify an arbitrary valid BCP 47 language tag for text attribution.
In character attribution dialogs the language list box (of the Western text font if CJK or CTL are enabled) is now a combo box with an edit field where the user can specify a valid BCP 47 language tag to define a text language attribute if the language she wants to assign is not available from the selectable list. The input is checked against a copy of the IANA language-subtag-registry (transformed to XML), provided either with the liblangtag package of your OS distribution, or the liblangtag shipped with LibreOffice on systems that do not provide one. The internal registry file distributed with LibreOffice 4.3 consists of data as of 2014-04-10.
For language tag details please see the For users section on the langtag.net web site.
And now jump to the release notes for more nifty features of LibreOffice 4.3 :-)