Add support for validating languages

Will validate against ISO 639-2 or ISO 639-3 depending on how long
the language field is. Otherwise will return that the language is
invalid.

Does not currently have any support for generic values like "Other".

csv_metadata_quality/app.py

@@ -43,6 +43,11 @@ def main(argv):
         # Fix: duplicate metadata values
         df[column] = df[column].apply(fix.duplicates)
 
+        # Check: invalid language
+        match = re.match(r'^.*?language.*$', column)
+        if match is not None:
+            df[column] = df[column].apply(check.language)
+
         # Check: invalid ISSN
         match = re.match(r'^.*?issn.*$', column)
         if match is not None: