Translation language respect current url (#734)

lib/core/nav/HeaderNav.js

@@ -33,11 +33,29 @@ class LanguageDropDown extends React.Component {
     const enabledLanguages = env.translation
       .enabledLanguages()
       .filter(lang => lang.tag !== this.props.language)
-      .map(lang => (
+      .map(lang => {
-        <li key={lang.tag}>
+        // build the href so that we try to stay in current url but change the language.
-          <a href={siteConfig.baseUrl + lang.tag}>{lang.name}</a>
+        let href = siteConfig.baseUrl + lang.tag;
-        </li>
+        if (
-      ));
+          this.props.current &&
+          this.props.current.permalink &&
+          this.props.language
+        ) {
+          href =
+            siteConfig.baseUrl +
+            this.props.current.permalink.replace(
+              `/${this.props.language}/`,
+              `/${lang.tag}/`
+            );
+        } else if (this.props.current.id && this.props.current.id !== 'index') {
+          href = siteConfig.baseUrl + lang.tag + '/' + this.props.current.id;
+        }
+        return (
+          <li key={lang.tag}>
+            <a href={utils.getPath(href, this.props.cleanUrl)}>{lang.name}</a>
+          </li>
+        );
+      });
     // if no languages are enabled besides English, return null
     if (enabledLanguages.length < 1) {
       return null;
@@ -131,6 +149,8 @@ class HeaderNav extends React.Component {
           <LanguageDropDown
             baseUrl={this.props.baseUrl}
             language={this.props.language}
+            current={this.props.current}
+            cleanUrl={this.props.config.cleanUrl}
             key="languagedropdown"
           />
         );