KStandardAction: add method for SwitchApplicationLanguage action creation

Summary:
When SwitchApplicationLanguage was added to the enum StandardAction,
a convenience method was defined to generate the action, but the
declaration was forgotten. No convenience method being made available
is both inconsistent with the rest of the API, also is the method
otherwise unused internally.

Reviewers: Frameworks, aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26873