Add english translation

src/Resources/contao/languages/en/modules.php

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+<?php
+
+$GLOBALS['TL_LANG']['MOD']['tfa_explanation'] = 'To activate two-factor authentication for your account please scan the QR code with a compatible app such as <strong>Google Authenticator</strong> (available for iOS and Android) and type in the generated code afterwards.';
+$GLOBALS['TL_LANG']['MOD']['tfa_activated'] = 'You have already activated two-factor authentication. Please keep your authenticator app at the ready when you log into Contao in the future.';
+$GLOBALS['TL_LANG']['MOD']['tfa_token'] = 'Code generated by your app';

src/Resources/contao/languages/en/tl_user.php

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+<?php
+
+$GLOBALS['TL_LANG']['tl_user']['tfa_title'] = 'Two-Factor Authentication';
+$GLOBALS['TL_LANG']['tl_user']['tfa_exception_invalid'] = 'The entered code is invalid.';