nextcloud/settings/ajax/setlanguage.php

<?php

$l = \OC::$server->getL10N('settings');

OC_JSON::checkLoggedIn();
OCP\JSON::callCheck();


// Get data
if( isset( $_POST['lang'] ) ) {
	$languageCodes=OC_L10N::findAvailableLanguages();
	$lang=$_POST['lang'];
	if(array_search($lang, $languageCodes) or $lang === 'en') {
		\OC::$server->getConfig()->setUserValue( OC_User::getUser(), 'core', 'lang', $lang );
		OC_JSON::success(array("data" => array( "message" => $l->t("Language changed") )));
	}else{
		OC_JSON::error(array("data" => array( "message" => $l->t("Invalid request") )));
	}
}else{
	OC_JSON::error(array("data" => array( "message" => $l->t("Invalid request") )));
}