fix: algolia search doesn't work well at i18n mode

Add lang facetFilters presets by following VuePress's i18n design logic

lib/default-theme/AlgoliaSearchBox.vue

@@ -15,28 +15,44 @@ export default {
   props: ['options'],
 
   mounted () {
-    this.initialize()
+    this.initialize(this.options, this.$lang)
   },
 
   methods: {
-    initialize () {
+    initialize (userOptions, lang) {
       Promise.all([
         import(/* webpackChunkName: "docsearch" */ 'docsearch.js/dist/cdn/docsearch.min.js'),
         import(/* webpackChunkName: "docsearch" */ 'docsearch.js/dist/cdn/docsearch.min.css')
       ]).then(([docsearch]) => {
         docsearch = docsearch.default
-        docsearch(Object.assign(this.options, {
-          debug: true,
-          inputSelector: '#algolia-search-input'
-        }))
+        const { algoliaOptions = {}} = userOptions
+        docsearch(Object.assign(
+          {},
+          userOptions,
+          {
+            inputSelector: '#algolia-search-input',
+            // #697 Make docsearch work well at i18n mode.
+            algoliaOptions: Object.assign({
+              'facetFilters': [`lang:${lang}`].concat(algoliaOptions.facetFilters || [])
+            }, algoliaOptions)
+          }
+        ))
       })
+    },
+
+    update (options, lang) {
+      this.$el.innerHTML = '<input id="algolia-search-input" class="search-query">'
+      this.initialize(options, lang)
     }
   },
 
   watch: {
+    $lang (newValue) {
+      this.update(this.options, newValue)
+    },
+
     options (newValue) {
-      this.$el.innerHTML = '<input id="algolia-search-input" class="search-query">'
-      this.initialize(newValue)
+      this.update(newValue, this.$lang)
     }
   }
 }