Make accesibility score 100 in Lighthouse

While we could fetch the lang attribute data from the gatsby config file, it's probably better to couple those with the particular page's Helmet as if we have a different language (i18n) we would need to be able to define that too.

src/pages/index.js

@@ -14,7 +14,7 @@ class BlogIndex extends React.Component {
 
     return (
       <Layout location={this.props.location}>
-        <Helmet title={siteTitle} />
+        <Helmet title={siteTitle} htmlAttributes={{ lang: 'en' }} />
         <Bio />
         {posts.map(({ node }) => {
           const title = get(node, 'frontmatter.title') || node.fields.slug

src/templates/blog-post.js

@@ -15,7 +15,10 @@ class BlogPostTemplate extends React.Component {
 
     return (
       <Layout location={this.props.location}>
-        <Helmet title={`${post.frontmatter.title} | ${siteTitle}`} />
+        <Helmet
+          title={`${post.frontmatter.title} | ${siteTitle}`}
+          htmlAttributes={{ lang: 'en' }}
+        />
         <h1>{post.frontmatter.title}</h1>
         <p
           style={{