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I go to phpBB Admin Control Panel (ACP) / Users and Groups /
(LEFT PANLE) Manage Users
[Find a member] noproblem [SUBMIT]

I get these errors at the top of the results page when I press [SUBMIT]
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/language/language.php on line 313: vsprintf(): Too few arguments
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 138: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3037)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 138: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3037)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 138: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3037)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 138: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3037)
Thank you for helping out

I also encountered the same situation on the new member registration page. I don't know how to fix it.
"[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/language/language.php on line 313: vsprintf(): Too few arguments"

Sorry, I can't reproduce the issue, the language pack don't seems to be the cause.

You should post in the phpBB support forum.