Hi,
I have over 5 years experience in Drupal all its versions (5, 6 and 7). I built some big sites like [login to view URL], [login to view URL] and [login to view URL], building cool features and integration with third-party services.
I have experience working in multi language sites and also building sub-themes. My suggestion is to use Boostrap as a responsive base theme ([login to view URL]).
Please feel free to contact me for more detail and questions.
Thanks!