go to the language section and change the link at the top to read 

http://www.example.com/admin/settings/dev-tools/languages

so you just add the dev-tools/ in there as you can see ^^

then you can add new keys, you choose the section you want ie forum or whatever in the drop down.. and then when you add the key just put 

name_of_key_like_this   and then the text of what it means below that 

you do not have to put the plugin name in the text key, the system will add that 

so if you add 

my_new_key   when you save it the system will add pluginname+my_new_key

so in the case of the image above you just select forumpro from the dropdown

and then choose add and then just put in topic_allow_files and whatever you want it to be below that and then save it

Hello dave, very thanks for the nice info, so i try to add this language key but it doesnt work

whats for a plugin is it? Group? Hint? Base?

I mean its the Group Plugin, but i add the key on Groups and Hint and not working ...

hint+group_info_access_by_invitation

base+questions_question_sex_value_16  this key dont work too! (Added on Base)

the first part of any key is the plugin name

so if you want to add lang for hint+group_info_access_by_invitation  then you choose 

hint from the drop down menu  and add  group_info_access_by_invitation and the value below it. thats it.  You do not have to use the plugin name other than in the drop down list.

like this 

if you get a parse error it just means that you waited too long and the token has expired so close the box and refresh the page. 

oh now i have understand! Very thank u for help!  I think dev-tools/languages must be linked as standart menu. Have a nice Day :)