wp_list_categories (and language files?) causing HTTP Error 500?

When the stattraq plugin is installed (and it appears to work fine) and the argument “show_count=1” is passed to wp_list_categories, an Error 500 occurs, which disappears if either is removed (the plugin or this parameter).

While this might be caused by stattraq, the same error occurs when the this parameter is specified (“show_count=1”) and language files (specifically languages/fr_FR.mo and languages/fr_FR.po) are installed. When this parameter is removed, the function proceeds correctly and the blog is translated. But apparently the presence of these language files is also a problem in and of itself: it is then impossible to enter the dashboard (Error 500) until the directory languages is removed (or renamed).

I am quite at a loss to have wordpress 2.9.1 work reliably just with a standard translation and using its standard functions.