TYPO3 Core

TYPO3 has a list of "known" languages set in TYPO3\CMS\Core\Localization\Locales.
To extend this list one can use something like this:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['localization']['locales']['user']['id'] = 'Indonesian';

For reading the translation of a word in this language, one can use:

$GLOBALS['TSFE']->sL('LLL:path/to/lang/file/lang.xml:langkey');

Problem¶

The user defined locales are not taken into account.
In TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController (line 4675):

$locales = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Localization\\Locales');

// Language is found. Configure it:
if (in_array($this->lang, $locales->getLocales())) {
    $this->languageDependencies[] = $this->lang;
    foreach ($locales->getLocaleDependencies($this->lang) as $language) {
        $this->languageDependencies[] = $language;
    }
}

There is no call to $locales->initialize() which would read the user defined settings:

$locales = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Localization\\Locales');
$locales->initialize();

// Language is found. Configure it:
if (in_array($this->lang, $locales->getLocales())) {
    $this->languageDependencies[] = $this->lang;
    foreach ($locales->getLocaleDependencies($this->lang) as $language) {
        $this->languageDependencies[] = $language;
    }
}