doc: Refer to the es and zh_CN translations.

* doc/guix.texi (Top): Mention Simplified Chinese and Spanish.

doc/guix.texi

@@ -108,10 +108,11 @@ package management tool written for the GNU system.
 @c TRANSLATORS: You can replace the following paragraph with information on
 @c how to join your own translation team and how to report issues with the
 @c translation.
-This manual is also available in French (@pxref{Top,,, guix.fr, Manuel de
-référence de GNU Guix}) and German (@pxref{Top,,, guix.de, Referenzhandbuch
-zu GNU Guix}).  If you would like to translate it in your native language,
-consider joining the
+This manual is also available in Simplified Chinese (@pxref{Top,,, guix.zh_CN,
+GNU Guix参考手册}), French (@pxref{Top,,, guix.fr, Manuel de référence de GNU
+Guix}), German (@pxref{Top,,, guix.de, Referenzhandbuch zu GNU Guix}), and
+Spanish (@pxref{Top,,, guix.es, Manual de referencia de GNU Guix}).  If you
+would like to translate it in your native language, consider joining the
 @uref{https://translationproject.org/domain/guix-manual.html, Translation
 Project}.