I'm using the tools/build_namespace_langs.py command in mwlib 0.11.2 with
an updated patch which better handles the $fallback option.  The current
patch to that file (in mwlib, not in this archive) is:

diff --git a/tools/build_namespace_langs.py b/tools/build_namespace_langs.py
index ea2ab15..4082b71 100755
--- a/tools/build_namespace_langs.py
+++ b/tools/build_namespace_langs.py
@@ -12,14 +12,15 @@ import sys

 from mwlib import namespace

-filename_rex = re.compile(r'^Messages(?P<lang>\w\w\w?)\.php$')
+filename_rex = re.compile(r'^Messages(?P<lang>\w\w\w?(?:_\w+)?)\.php$')
+fallback_rex = re.compile(r'^\$fallback\s*=\s*["\'](?P<lang>\w\w\w?(?:-\w+)?)["\']')
 start_names_rex = re.compile(r'^\$namespaceNames = array\($')
 start_aliases_rex = re.compile(r'^\$namespaceAliases = array\($')
 const2name_rex = re.compile('^\\s*(?P<const>NS_\w+)\\s*=>\\s*["\'](?P<name>\S+)["\']\s*,?\s*$')
 name2const_rex = re.compile('^\\s*["\'](?P<name>\S+)["\']\\s*=>\\s*(?P<const>NS_\w+)\s*,?\s*$')
 end_rex = re.compile(r'^\);$')

-def parse_namespace_names(fn):
+def parse_namespace_names(fn, msgsdir):
     ns2name = {}
     started = False
     for line in open(fn, 'rb'):
@@ -27,6 +28,9 @@ def parse_namespace_names(fn):
             if start_names_rex.match(line):
                 started = True
             else:
+                mo = fallback_rex.match(line)
+                if mo is not None:
+                    ns2name['$fallback'] = mo.group('lang').lower()
                 continue
         if end_rex.match(line):
             break
@@ -43,7 +47,15 @@ def parse_namespace_names(fn):
         ns2name[nsnum] = name
     else:
         # no namespace names found in this file
-        return None
+        if '$fallback' in ns2name:
+            fallback = ns2name['$fallback']
+            mo = 'Messages'+(fallback.capitalize().replace('-','_'))+'.php'
+            mo = os.path.join(msgsdir, mo)
+            ns2name = parse_namespace_names(mo, msgsdir)
+            if ns2name:
+                ns2name['$fallback'] = fallback
+        else:
+            return None
     return ns2name

 def parse_namespace_aliases(fn):
@@ -76,8 +88,9 @@ def get_ns_list(names, aliases):
     for name, nsnum in aliases.items():
         try:
             v = names[nsnum]
-        except KeyError:
-            return None
+        except KeyError, e:
+            names[nsnum] = name
+            continue
         if isinstance(v, unicode):
             names[nsnum] = (v, name)
         else:
@@ -85,7 +98,7 @@ def get_ns_list(names, aliases):

     proj_talk_name = names.get(namespace.NS_PROJECT_TALK, u'%s_talk')
     try:
-        return [names[nsnum] for nsnum in namespace._lang_ns_data_keys] + [proj_talk_name]
+        return [names.get(nsnum,None) for nsnum in namespace._lang_ns_data_keys] + [proj_talk_name]
     except KeyError, e:
         return None

@@ -97,24 +110,38 @@ def main(argv):
     assert os.path.isdir(msgsdir), '%r is not a directory' % msgsdir

     lang_ns_data = {}
+    lang_fallback = {}
     for fn in os.listdir(msgsdir):
         mo = filename_rex.match(fn)
         if mo is None:
             continue
-        lang = mo.group('lang').lower()
+        lang = mo.group('lang').lower().replace('_','-')
         if lang == 'en':
             # English is special: aliases are handled by other means etc.
             continue
         p = os.path.join(msgsdir, fn)
-        names = parse_namespace_names(p)
+        names = parse_namespace_names(p, msgsdir)
         if not names:
             continue
+        lang_fallback[lang] = names.get('$fallback', 'en')
         lst = get_ns_list(names, parse_namespace_aliases(p))
         if lst:
             lang_ns_data[lang] = lst

     lang_ns_data['en'] = [u'Talk', u'User', u'User_talk', (u'File', u'Image'), (u'File_talk', u'Image talk'), u'MediaWiki', u'MediaWiki_talk', u'Template', u'Template_talk', u'Help', u'Help_talk', u'Category', u'Category_talk', u'Special', u'Media', u'%s_talk']
-
+
+    # fill in missing entries from fallback languages
+    def fill_in_missing(lang):
+        data = lang_ns_data[lang]
+        if not(None in data): return
+        fallback = lang_fallback.get(lang, 'en')
+        fill_in_missing(fallback)
+        for i in xrange(len(data)):
+            if data[i] is None:
+                data[i] = lang_ns_data[fallback][i]
+    for lang in lang_ns_data.keys():
+        fill_in_missing(lang)
+
     s = ['lang_ns_data = {']
     for lang in sorted(lang_ns_data.keys()):
         s.append("'%s': %r," % (lang, lang_ns_data[lang]))