Could it be that the lang.php files for templates are not loaded?
http://www.dokuwiki.org/devel:templates says that templates can be localized (just like plugins) withe the following directory structure:
lang/<lang code>/lang.php
But I think those files are not loaded into the global $lang array. (at least not for templates). The monobook template loads the file manually in its main.php
//get needed language array
include DOKU_TPLINC."lang/en/lang.php";