Default language to English instead of crashing when browser has no language setting

[jperals]

I believe this fixes #316, which seems to happen when loading the site without any preferred language set in the browser (and without a language set in the location hash either).

Apparently, in this case webL10n was setting the language to an empty string, and then in supportsLang passing a selector like #lang option[value=] to jQuery was causing an error that basically was causing the other symptoms described in the aforementioned issue.

I just had to add a small extra check in a condition because the rest of the code in foopy.js already takes care of setting en as default —that is, if it doesn't crash.

Still, this could also had been fixed in other ways, for example by changing line 896 of l10n.js like this:

var lang = document.documentElement.lang || navigator.language || "en";

but I liked the fact that l10n.js seems to be language-neutral in the sense that it doesn't seem to enforce any default language itself, and preferred to keep it like that.

[jdm]

Thanks for finding the underlying issue!