Zabbix 2.0.x web frontend translation system is rotten. So, some strings are not translated.

In frontends/php/include/defines.inc.php
define('REPORT4_WEEKLY_DATE_FORMAT', _('d M Y H:i'));
is deined.

In frontends/php/report4.php
return zbx_date2str(REPORT4_WEEKLY_DATE_FORMAT,$t);
calling.

In frontends/php/locale/ja/LC_MESSAGES/frontend.po
#: include/defines.inc.php:911
#: include/views/configuration.services.edit.php:132
#: include/views/configuration.services.edit.php:133 report3.php:162
msgid "d M Y H:i"
msgstr "Y/m/d H:i"

But, it can not display translated string.

I think that it may depends on PHP feature. But you stop to use define(...) in other php file or use hard cording.