Add OctoPrint.options.locale and if set send X-Locale header on requests

See #1593

docs/jsclientlib/base.rst

@@ -14,6 +14,10 @@
    ``OctoPrint.options.baseurl``
        The base URL of the OctoPrint API
 
+   ``OctoPrint.options.locale``
+       The locale to set in ``X-Locale`` headers on API requests. Useful for API
+       endpoints that might return localized content.
+
 .. js:data:: OctoPrint.plugins
 
    Registration of client library components provided by plugins.

src/octoprint/static/js/app/client/base.js

@@ -46,7 +46,8 @@
 
     OctoPrint.options = {
         "baseurl": undefined,
-        "apikey": undefined
+        "apikey": undefined,
+        "locale": undefined
     };
 
     OctoPrint.plugins = {};
@@ -65,6 +66,10 @@
         var headers = $.extend({}, additional);
         headers["X-Api-Key"] = OctoPrint.options.apikey;
 
+        if (OctoPrint.options.locale !== undefined) {
+            headers["X-Locale"] = OctoPrint.options.locale;
+        }
+
         return headers;
     };