I originally thought maybe you could just add it to the db or just add it to a form option list in the php file, but thats not how its setup.  It would take more than just a few lines of code. 

Have a look at ow_system_plugins/base/views/controllers/flags_index.html

<tr><th style="width: 15%">{text key="base+flag_spam"}</th><th style="width: 15%">{text key="base+flag_offence"}</th><th style="width: 15%">{text key="base+flag_illegal"}</th></tr>

now you could certainly just add a new base key lang and add it so it would show like this here in the html file   {text key="base+flag_myspecialflag"}

but then what do you with it, you would have the write the code to process it on the list and also save it to the db.  It appears to be js based and i suck at js.  But the php file is not what i expected either.  Since it is in a loop you could test it out and see what happens maybe it will do it all for you after just adding the html, i have not yet tested or looked closely enough to know for sure. 

the other thing i might want to mention, is that if you are to the point of having to isolate flag types, maybe its time to just get rid of that member. 

first you add the base lang key    i added  flag_abusive   - it has to start with flag_ ok  and it will wrap just so you know if you make it too long... 

i made mine  

Abusive to others  

 and it wraps 

then you go to ow_system_plugins/base/views/controllers/flags_index.html

and add the code to the list that is there 

<!-- added abusive -->
<th style="width: 15%">

{text key="base+flag_abusive"}

</th>

then save that and clear your cache. 

then go to ow_system_plugins/base/components/flag.php 

and you will see this code 

        $element->setOptions(array(           

 'spam' => OW::getLanguage()->text('base', 'flag_spam'),           

 'offence' => OW::getLanguage()->text('base', 'flag_offence'),           

 'illegal' => OW::getLanguage()->text('base', 'flag_illegal'))

                    );

add abusive to the array but remember to move the last closeing ) to your new line and then add a comma to the prev line,  so its like this 

//added abusive 

        $element->setOptions(array(            

'spam' => OW::getLanguage()->text('base', 'flag_spam'),            

'offence' => OW::getLanguage()->text('base', 'flag_offence'),           

 'illegal' => OW::getLanguage()->text('base', 'flag_illegal'),           

 'abusive' => OW::getLanguage()->text('base', 'flag_abusive'))       

 );

and whallahhh its done... 

enjoy.. :)

I applied it but I do not see any space to change the languageI added the required code to flag.phpThere is an option on the site popup screen.Does not appear in language pack