Remove redundant languages from snowflake page

https://snowflake.torproject.org/ offers me en, en_GB, and en_US in the language drop-down menu. We should have one English, not three.

More generally, I bet we have a policy somewhere about which languages we want to be sure to include when we have translated something, and we should see if we're missing any of those; and also I hope we have a policy about which languages to not include (and ideally we should go back to transifex and remove those from the set that people can translate, to avoid giving people the impression that they will be doing something useful if they e.g. translate en_US to en_GB).

I'm sorry I don't know more -- I bet Emma or Antonela or Gus will know more about whether we have languages policies and habits in place. :)