.lang files are dependant of the language file being used in the case of phpBB2 Forums these files are used so that people using various languages can read stories etcetera using captions and so on in their own language. Here is and English lang example (it is actually a php file
//
// Index
//
$lang['Admin'] = 'Administration';
$lang['Not_admin'] = 'You are not authorised to administer this board';
$lang['Welcome_phpBB'] = 'Welcome to phpBB';
$lang['Admin_intro'] = 'Thank you for choosing phpBB as your forum solution. This screen will give you a quick overview of all the various statistics of your board. You can get back to this page by clicking on the <u>Admin Index</u> link in the left pane. To return to the index of your board, click the phpBB logo also in the left pane. The other links on the left hand side of this screen will allow you to control every aspect of your forum experience. Each screen will have instructions on how to use the tools.';
$lang['Main_index'] = 'Forum Index';
$lang['Forum_stats'] = 'Forum Statistics';
$lang['Admin_Index'] = 'Admin Index';
$lang['Preview_forum'] = 'Preview Forum';
$lang['Click_return_admin_index'] = 'Click %sHere%s to return to the Admin Index';
$lang['Statistic'] = 'Statistic';
$lang['Value'] = 'Value';
$lang['Number_posts'] = 'Number of posts';
$lang['Posts_per_day'] = 'Posts per day';
$lang['Number_topics'] = 'Number of topics';
$lang['Topics_per_day'] = 'Topics per day';
$lang['Number_users'] = 'Number of users';
$lang['Users_per_day'] = 'Users per day';
$lang['Board_started'] = 'Board started';
$lang['Avatar_dir_size'] = 'Avatar directory size';
$lang['Database_size'] = 'Database size';
$lang['Gzip_compression'] ='Gzip compression';
$lang['Not_available'] = 'Not available';
$lang['ON'] = 'ON'; // This is for GZip compression
$lang['OFF'] = 'OFF';
This is the Russian verion of the same file.
//
// Index
//
$lang['Admin'] = 'Àäìèíèñòðèðîâàíèå';
$lang['Not_admin'] = 'Ó âàñ íåò ïðàâ íà àäìèíèñòðèðîâàíèå';
$lang['Welcome_phpBB'] = 'Äîáðî ïîæàëîâàòü íà phpBB';
$lang['Admin_intro'] = 'Ñïàñèáî çà âûáîð phpBB â êà÷åñòâå ðåøåíèÿ äëÿ âàøèõ ôîðóìîâ. Íà ýòîé ñòðàíèöå äàí êðàòêèé îáçîð ðàçëè÷íûõ âîçìîæíîñòåé ýòîé äîñêè îáúÿâëåíèé. Âåðíóòüñÿ íà ýòó ñòðàíèöó âû ìîæåòå, ù¸ëêíóâ íà ññûëêó <u>Ãëàâíàÿ ñòðàíèöà</u> â ëåâîé ïàíåëè. Äëÿ ïåðåõîäà íà ñïèñîê ôîðóìîâ ù¸ëêíèòå ïî ëîãîòèïó phpBB òàêæå â ëåâîé ïàíåëè. Îñòàëüíûå ññûëêè â ëåâîé ÷àñòè ýòîãî ýêðàíà ïîçâîëÿò âàì óïðàâëÿòü âñåìè àñïåêòàìè âàøèõ ôîðóìîâ, íà êàæäîì ýêðàíå áóäóò äàíû èíñòðóêöèè ïî èñïîëüçîâàíèþ.';
$lang['Main_index'] = 'Ñïèñîê ôîðóìîâ';
$lang['Forum_stats'] = 'Ñòàòèñòèêà Ôîðóìîâ';
$lang['Admin_Index'] = 'Ãëàâíàÿ ñòðàíèöà';
$lang['Preview_forum'] = 'Ïðåäâàðèòåëüíûé ïðîñìîòð ôîðóìà';
$lang['Click_return_admin_index'] = '%sÂåðíóòüñÿ íà ãëàâíóþ ñòðàíèöó àäìèíèñòðàòîðñêîãî ðàçäåëà%s';
$lang['Statistic'] = 'Ñòàòèñòèêà';
$lang['Value'] = 'Çíà÷åíèå';
$lang['Number_posts'] = 'Êîë-âî ñîîáùåíèé';
$lang['Posts_per_day'] = 'Ñîîáùåíèé â äåíü';
$lang['Number_topics'] = 'Êîë-âî òåì';
$lang['Topics_per_day'] = 'Òåì â äåíü';
$lang['Number_users'] = 'Êîë-âî ïîëüçîâàòåëåé';
$lang['Users_per_day'] = 'Ïîëüçîâàòåëåé â äåíü';
$lang['Board_started'] = 'Äàòà çàïóñêà';
$lang['Avatar_dir_size'] = 'Ðàçìåð äèðåêòîðèè ñ àâàòàðàìè';
$lang['Database_size'] = 'Îáú¸ì ÁÄ';
$lang['Gzip_compression'] ='ñæàòèå Gzip';
$lang['Not_available'] = 'Íåäîñòóïíî';
$lang['ON'] = 'ÂÊË'; // This is for GZip compression
$lang['OFF'] = 'ÂÛÊË';
Hope that sheds some light on setting a langauage preference and if an interpretation is there it is used otherwise a default language usually English is used.
