On http://www.phpbb.com/mods/faq.php#validate" target="_blank
It says

phpBB's Language System
Your MOD must use phpBB's Language system. This means putting all words inside the $lang array $lang[]. Since phpBB is used by people around the world, it needs to be translated easily. If translators have to hunt through code to find what needs to be translated it makes it hard. If you use $lang["> then translators just have to translate those making less work for them.

Where it says

If you use $lang[">

It should be

If you use $lang[]

or

If you use $lang[""]