v1.7.1 adds the ability to filter tweets on language. You can use from the command line, with one or more --lang arguments. So for example this will filter tweets that mention *Paris* or *Madrid* that are written in *French* or *Spanish*.
twarc filter paris,madrid --lang es --lang fr
You can also use programmatically:
python
for tweet in t.filter('paris,madrid', lang=['es', 'sp']):
print(tweet['full_text'])
Thanks for the idea tugrulz and for your patience!