.desktop file

In order to find your language support plugin, KDE needs a .desktop file with some meta information.

X-KDE-Library=... this must be the same as the name of your library, i.e. look into the CMakeLists.txt for your kde4_add_plugin line and use the same name
X-KDE-PluginInfo-Name=... this must be a unique name that is used to reference and find your plugin.

After installing your plugin, make sure to run kbuildsycoca4. For testing, try duchainify or ktraderclient.

the plugin

A basic plugin's header looks like this:

#include <interfaces/iplugin.h>
#include <language/interfaces/ilanguagesupport.h>

class KDevTestLangPlugin : public KDevelop::IPlugin, public KDevelop::ILanguageSupport
{
    Q_OBJECT
    Q_INTERFACES( KDevelop::ILanguageSupport )
    public:
        explicit KDevTestLangPlugin( QObject* parent, const QVariantList& args = QVariantList() );
        
        virtual KDevelop::ParseJob* createParseJob(const KDevelop::IndexedString& url);
        virtual QString name() const;
};

An implementation could look like this:

#include "kdevtestlangplugin.h"
#include <KPluginFactory>
#include <KAboutData>

#include "testlangparsejob.h"

K_PLUGIN_FACTORY(KDevTestLangSupportFactory, registerPlugin<KDevTestLangPlugin>(); )
K_EXPORT_PLUGIN(KDevTestLangSupportFactory(KAboutData("kdevtestlangsupport",0, ki18n("TestLanguage Support"), "0.1", ki18n("Support for Some Test Language"), KAboutData::License_GPL)))

using namespace KDevelop;

KDevTestLangPlugin::KDevTestLangPlugin(QObject* parent, const QVariantList& )
     : IPlugin( KDevTestLangSupportFactory::componentData(), parent )
     , ILanguageSupport()
{
    // NOTE: this is important!
    KDEV_USE_EXTENSION_INTERFACE(ILanguageSupport)
}

ParseJob* KDevTestLangPlugin::createParseJob(const IndexedString& url)
{
    return new TestLangParseJob(url, this);
}

QString KDevTestLangPlugin::name() const
{
    // some unique id for your language
    return "testlang";
}