Make i18n for 'callout-firefoxos' link in homepage.html

Even if we visit [the homepage with setting ja locale](https://developer.mozilla.org/ja/), the link of "Firefox OS をビルドする (en-us: "Build Firefox OS")" is not localized. Therefore the link always jumps to `https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS`. This is the serious globalization issue.

This bug is caused with that Kuma has not made it i18n with using `devmo_url()`. So this pull request fix it.