Use correct language attribute in html tag

Ghost User
2019-03-27T19:18:56+01:00

### One-sentence description The `lang` attribute of the root html element is hardcoded to "en", this can be set to the LANGUAGE_CODE ### Why? The [specification recommends it](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang#Specifications). ### Current implementation The value is hardcoded to `"en"` ### Suggested implementation Set the lang attribute to the current language code