Fixes for a regression introduced by #4221

#4221 was incorrectly using str_replace() to remove the default language code when required. As str_replace() acts globally on the subject, the language code was removed also out of context (e.g. when part of an alias).

With this PR I use instead preg_replace() with the limit of one substitution. As the /language-code/ string to be replaced appears between two slashes and just after the host name (eg. http://example.com/en/) this should guarantee the correct replacements of the language code only.

I took the occasion to make a bit of clean-up by aggregating some redundantly duplicated code