In short, you should leave the URLs in links to other articles of www.gnu.org as they appear in the English text.
These days www.gnu.org uses HTTP language negotiation to provide the most preferred translation available according to user’s browser settings. The texts of articles use generic URLs like ‘/directory/article.html’ (note no language suffix). When the visitor follows such links, www.gnu.org chooses the best translated version, or the English version if there is no suitable translation.
Once upon a time, there was a practice to link to the respective translation (‘/directory/article.lang.html’) when available. You shouldn’t do this any more. First, new translations are added, and occasionally even removed, and timely updating the links in all existing translations is not feasible. Second, and more important, visiting a translation doesn’t really imply that its language is the most preferable one.
For instance, let us imagine that visitor’s native language is Serbian, and she can also understand Bulgarian. Then (as of Sep 2015) the best version of Enforcing the GPL is Bulgarian; however, that page links to the Free Software Definition, which is available both in Serbian and in Bulgarian. If the Bulgarian translation of the announcement linked to the Bulgarian version of the definition, the visitor would be directed to a wrong translation.