go to edit language and click save this will regenerate the language cache this has been known to fix this its a 50/50 chance

thanks Joseph, i got the following message after debug

"Message: opendir(/xxxxx/xxxxxxx/public_html/ow_pluginfiles/admin/languages/import/base/) [function.opendir]: failed to open dir: No such file or directory

Try to manually import the lang.zip file in the link

http://site.com/admin/dev-tools/languages

where to import the english zip file ? i did edit and save the language joseph but still have the same result

why i cannot upload language pack? i get empty or invalid language pack message, is there something missing?

any idea how to fix this? i think it's not language problem

It's like a magic, i only change the .htaccess to new version and then i can upload the language pack and fix, thanks everyone