add lang element to pkgdown template

This is a first step towards addressing #205, but there is still a lot
more work that needs to go into it. Because I'm pretty sure the tests
will not pass, I'm going to skip the continuous integration for now.

[skip ci]

R/utils-yaml.R

@@ -162,6 +162,7 @@ create_pkgdown_yaml <- function(path) {
   yaml <- whisker::whisker.render(yaml, 
     data = list(
       # Basic information
+      lang    = if (is.null(usr$lang)) NULL else siQuote(usr$lang),
       version = siQuote(utils::packageVersion("sandpaper")),
       config  = siQuote(path_config(path)),
       title   = siQuote(usr$title),

inst/templates/pkgdown-yaml-template.txt

@@ -3,6 +3,7 @@
 # If you want to make changes, please edit {{ config }}
 # ------------------------------------------------------------------ information 
 title: {{ title }} # needed to set the site title
+lang: {{ lang }}{{^ lang }}'en'{{ /lang }}
 home:
   title: Home
   strip_header: true