In NAV2009 and earlier, GLOBALLANGUAGE(0) would set the language in NAV to the “default” language of the classic client.
This undocumented feature does not work in NAV 2013 anymore. It will return an error saying:
Exception of type 'Microsoft.Dynamics.Nav.Types.Exceptions. NavNotSupportedLanguageException' was thrown.
So you must instead set the language ID to the correct value. Use codeunit 43 for this.