Sphinx: Use newly-added READTHEDOCS_LANGUAGE env var

Upstream ReadTheDocs added it in readthedocs/readthedocs.org#5941
so that we don't have to define it ourselves.

conf.py

@@ -40,7 +40,7 @@ if env_tags != None:
        tags.add(tag.strip())
 
 # Language / i18n
-language = os.getenv('RTD_LANGUAGE', 'en')
+language = os.getenv('READTHEDOCS_LANGUAGE', 'en')
 is_i18n = tags.has('i18n')
 
 exclude_patterns = ['_build']