TranslationHelper::setL18nFolderpath() 

\nn\t3::TranslationHelper()->setL18nFolderpath($l18nFolderpath); 

Sets the current folder in which the translation files are cached. The idea is to translate the translated texts for backend modules only once and then save them in the extension folder. From there they are then deployed to GIT.

Default is typo3conf/l10n/nnhelpers/

$translationHelper->setL18nFolderpath('EXT:myext/Resources/Private/Language/');

public function setL18nFolderpath($l18nFolderpath) {
	$this->l18nFolderpath = $l18nFolderpath;
	return $this;
}