To use the locallang values in our extension controller, use the following function
TYPO3\CMS\Extbase\Utility\LocalizationUtility::translate('your.label.key','YourExtensionName', $optionalArrayWithParamsToReplaceInLocalLangString);
Example
$myvar = 'testvar';
TYPO3\CMS\Extbase\Utility\LocalizationUtility::translate('mylabel','myext',array($myvar));
and in locallang.xlf, use
<trans-unit id="mylabel">
<source>Your variable is %s </source>
</trans-unit>