Respect element lang attribute if present

Should parts of the document specify language, use it for grammar
checking.

online-editor.js

@@ -45,7 +45,7 @@ async function besProof(el) {
                 x.markup ? { markup: x.text } : { text: x.text }
               )
             }),
-            language: 'sl',
+            language: el.lang ? el.lang : 'sl',
             level: 'picky'
           }
           const request = new Request(besUrl, {