This is a bug in the translation plugin (from http://wiki.splitbrain.org/plugin:translation
When viewing a translation, there is a bug in the code for that builds links to other translations. The link to the original page (the English one, for example) refers to the same namespace as the current page, which can for example be "fr" when viewing a French translation. This only happens when enabling the plugin all over the site, because then "$this->tns" in the syntax_plugin_translation plugin refers to nothing.
In the function _buildTransLink(), the following code is executed when $lng is empty (so when requesting the link to the page in the original language):
$link = $this->tns.$idpart;
$name = $conf['lang'];
However, when the plugin isn't enabled only in a specific namespace, but all over the site, "$this->tns" is empty, and simply the string $idpart will be given to the "html_wikilink($link,$name);" call underneath. But apparently (although I could be wrong), html_wikilink automatically places an ID without a namespace in the current namespace.
So, for example, when viewing the page "fr:foo", the links to the English (original), German and French translation will respectively be "fr:foo", "de:foo" and "fr:foo", instead of "foo", "de:foo" and "fr:foo".