Support Cyrillic and Latin character sets for Serbian.

See collective/plone.app.locales#326

Let 'sr' be general Serbian. Can be Latin or Cyrillic.
You can influence which character set is used, by choosing one of three values in an environment variable:

```
zope_i18n_allowed_languages sr
zope_i18n_allowed_languages sr@Cyrl
zope_i18n_allowed_languages sr@Latn
```

Default character set in Plone 5.2 is Cyrillic, but this may change to Latin in 6.0.

news/326.feature

@@ -0,0 +1,3 @@
+Support Cyrillic and Latin character sets for Serbian.
+See `locales issue 326 <https://github.com/collective/plone.app.locales/issues/326>`_.
+[maurits, fredvd]

plone/i18n/locales/languages.py

@@ -4,6 +4,7 @@
 from plone.i18n.locales.interfaces import IMetadataLanguageAvailability
 from zope.interface import implementer
 
+import os
 import six
 
 
@@ -469,6 +470,8 @@ class MetadataLanguageAvailability(LanguageAvailability):
         u'flag': u'/++resource++country-flags/al.gif',
     },
     u'sr': {
+        # Note: we support two character sets for this language.
+        # See zope_i18n_allowed_languages below.
         u'native': 'српски',
         u'name': 'Serbian',
         u'flag': u'/++resource++country-flags/cs.gif',
@@ -562,6 +565,23 @@ class MetadataLanguageAvailability(LanguageAvailability):
     },
 }
 
+# Character sets are a thing now.
+# See https://github.com/collective/plone.app.locales/issues/326
+# At the moment only for Serbian.
+_zope_i18n_allowed_languages = os.environ.get("zope_i18n_allowed_languages", "")
+if 'sr@Latn' in _zope_i18n_allowed_languages:
+    _languagelist[u'sr'] = {
+        u'native': 'Srpski',
+        u'name': 'Serbian (Latin)',
+        u'flag': u'/++resource++country-flags/cs.gif',
+    }
+elif 'sr@Cyrl' in _zope_i18n_allowed_languages:
+    _languagelist[u'sr'] = {
+        u'native': 'српски',
+        u'name': 'Serbian (Cyrillic)',
+        u'flag': u'/++resource++country-flags/cs.gif',
+    }
+
 # convert the utf-8 encoded values to unicode
 for code in _languagelist:
     value = _languagelist[code]