Make translate.rb plugin fallback to English by default

Live preview: http://bitcointest1.us.to/en/

This pull requests allows {% translate %} to return the English string if no translated string is available. This can simplify updating translations and layouts in some cases.

This pull req also displays more English strings in outdated translations (e.g. some important texts still missing for some languages). This allows to reduce the number of "language-specific" content in templates and reduce the complexity of adding new content in some cases. Although displaying English strings in translations is not ideal, it appears better to me in some cases than serving incomplete content.