Allow setting of default language through config

Setting of locale in controller extracted to Localized concern,
the doorkeeper authorized applications controller moved under
custom namespace with inclusion of Localized, which resolves the
"it sometimes appears in a different random language" bug

.env.production.sample

@@ -25,6 +25,9 @@ OTP_SECRET=
 # Only allow registrations with the following e-mail domains
 # EMAIL_DOMAIN_WHITELIST=example1.com|example2.de|etc
 
+# Optionally change default language
+# DEFAULT_LOCALE=de
+
 # E-mail configuration
 SMTP_SERVER=smtp.mailgun.org
 SMTP_PORT=587