support HTML lang tag

[casperdcl]

A lang attribute is considered essential

[chrisrhymes]

Thanks for your pull request! Please can you change the below so there is a default lang for users that haven't set it in the _config.yml file.

{% if site.lang %}
 lang="{{ site.lang }}"{% endif %}

to

lang="{{ site.lang | default: 'en' }}"

[casperdcl]

Yeah I wasn't too sure about setting this default because it may potentially change behaviour... e.g. a French site usually viewed on a browser configured to assume French by default will now be forced to assume English.

What do you think?

[chrisrhymes]

That's a very good point!