Add 'xx' language ID for multi-language support

Allows models to specify their language ID as 'xx'.

spacy/lang/xx/__init__.py

@@ -0,0 +1,26 @@
+# coding: utf8
+from __future__ import unicode_literals
+
+
+from ..tokenizer_exceptions import BASE_EXCEPTIONS
+from ...language import Language
+from ...attrs import LANG
+from ...util import update_exc
+
+
+class MultiLanguageDefaults(Language.Defaults):
+    lex_attr_getters = dict(Language.Defaults.lex_attr_getters)
+    lex_attr_getters[LANG] = lambda text: 'xx'
+
+    tokenizer_exceptions = update_exc(BASE_EXCEPTIONS)
+
+
+class MultiLanguage(Language):
+    """Language class to be used for models that support multiple languages.
+    This module allows models to specify their language ID as 'xx'.
+    """
+    lang = 'xx'
+    Defaults = MultiLanguageDefaults
+
+
+__all__ = ['MultiLanguage']