/usr/share/phpbb3/www/language/en_us/email/user_activate.txt is in phpbb3-l10n 3.0.10-4+deb7u3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | Subject: Reactivate your account
Hello {USERNAME},
Your account on "{SITENAME}" has been deactivated, most likely due to changes made to your profile. In order to reactivate your account you must click on the link below:
{U_ACTIVATE}
{EMAIL_SIG}
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