Fix filename lang metadata regex

Make the lang metadata only match the two possible languages in the
blog.
Without this, filenames, even from static files, containing '-' throwed
a 'STATIC_LANG_SAVE_AS' critical error.
Now they can have '-' in the name as long as it isn't '-en' or '-br'.

pelicanconf.py

@@ -32,7 +32,7 @@ EXTRA_PATH_METADATA = {
     }
 
 # Before the dash, use to match translations. After dash assign to language.
-FILENAME_METADATA = '(?P<trans_id>.*)-(?P<lang>.*)'
+FILENAME_METADATA = '(?P<trans_id>.*)-(?P<lang>br|en)'
 ARTICLE_TRANSLATION_ID = 'trans_id'
 
 PLUGIN_PATHS = ['pelican-plugins']