Twig will always use gettext, legacy will use legacy. Twig does not
work with legacy so new ui would break if you would not also set this
option. Instead, just switch the system in use based on the usenewui
config variable.

bugfix: $langPath won't be defined if an exception is thrown by the method where we get its value from.

Use \SimpleSAML\Logger in the new Localization class, and fix general Exception being called from a namespace.