fix language prefix handling for drupal:path used with path aliases

I know that drupal:path and drupal:path:wildcard stuff is hard to understand. But there're multiple mistakes in your examples.

First "drupal:path = foo/bar% >>> my_theme_2" is not a valid rule. Path placeholders (aka wildcards) could only be used as complete part of a path like "foo/bar/%".
Second "drupal:path = foo/bar/% >>> my_theme_2" could never be reached because everything that matches this rule also matches "drupal:path = foo/bar >>> my_theme_1". You have to exchange both rules.
That's all described in the "properties explained" documentation for drupal:path.

"drupal:path = %foo/bar >>> my_theme_1" declares a wildcard named "foo" and matches "ANYTHING_ALPHANUMMERIC/bar". Wildcards could be accessed later in the rule chain using "drupal:path:wildcard foo = SOMETHING >>> another_theme".

Regular expressions have nothing to do with drupal:path:wildcard. They could be used for nearly any property and are activated using the "~" operator. See the "Operators explained" section of the documentation.

I recommend that you activate the ThemeKey Debug module and set "Trace rule switching" to see what's going on.

Now the last part. I never tested ThemeKey with language prefix paths. That's an interesting point. I guess that the prefix gets removed for the rule check. (I have to verify that later.) If this is the case you can cascade rules:

   drupal:path = foo/bar/%
      locale:language = de >>> german_sub_section_theme
      locale:language = en >>> english_sub_section_theme
      system:dummy = dummy >>> any_other_language_sub_section_theme
   drupal:path = foo/bar
      locale:language = de >>> german_master_theme
      locale:language = en >>> english_master_theme
      system:dummy = dummy >>> any_other_master_theme

Thanks for the feedback. Maybe it makes sense to strip the language prefix for drupal:path. For backwards compatibility it should be configurable. Turned on by default on a fresh installation but turned off on updates.

Meanwhile you could reduce your rules to two:

drupal:path = foo/bar/% >>> mytheme
drupal:path = %/foo/bar/% >>> mytheme

If you want to match "foo/bar" as well, you can replace the rules above by

drupal:path = foo/bar >>> mytheme
drupal:path = %/foo/bar >>> mytheme

These include the ones from above!

Wow, drupal 7's flexible language negotiation caused complicated code to fix this issue ;-)

We don't need a configuration option. The patch should solve all cases!