Special-case english by making it always 100% translated

Solves #1493. To avoid the ugly negative 100% from english, we make translation metadata for the english language pack always have 100% translations. Now it sums up to 0% additional translations.

![english](https://f.cloud.github.com/assets/191955/2213531/9eeb3136-99c1-11e3-9428-21305f33b302.png)