This is a fresh install, new board - download package from phpBB.com. Danish language pack installed then . . .

When accessing ACP> System> Language packs this error appears:

[phpBB Debug] PHP Notice: in file /includes/acp/acp_language.php on line 1123: file_exists() [function.file-exists]: open_basedir restriction in effect. File(./../language/index.htm/iso.txt) is not within the allowed path(s): (/customers/pelle.pl/pelle.pl:/var/www/diagnostics:/usr/share/php)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 150: Cannot modify header information - headers already sent by (output started at /includes/functions.php:3493)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 152: Cannot modify header information - headers already sent by (output started at /includes/functions.php:3493)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 153: Cannot modify header information - headers already sent by (output started at /includes/functions.php:3493)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 154: Cannot modify header information - headers already sent by (output started at /includes/functions.php:3493)

so can't install a custom language pack.