Issue10

Now I've introduced i18n mechanism for the front-end, and translated (maybe) all messages in it.

I ensured to use same terms with Zabbix for both English & Japanese messages.
(I referred frontends/php/locale/ja/LC_MESSAGES/frontend.po in zabbix).

May I merge it to master?
(This modification conflicts with Yamato-san's work in issue24-3.)