When user (admin) tries to update from phpBB 3.0 to 3.1 and he uses different default language than English and that (his default) language is not available in installation pack, he can get the following error:
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/user.php on line 204: include(./language/lang_name/common.php): failed to open stream: No such file or directory
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/user.php on line 204: include(): Failed opening './language/lang_name/common.php' for inclusion (include_path='.;C:/php/pear')
Language file ./language/lang_name/common.php couldn't be opened.
Now I know it can be solved by uploading of language pack to language folder, or renaming of the en language pack to missing language name, but wouldn't be better to check if installation pack contains admin's default language pack and if not, then reset admin's language setting to English language?