UABB comes with a POT which can be loaded in Poedit for translating the strings. This file is located in 『bb -ultimate-addon/languages/』
Open Poedit, Go to File -> From POT/PO file.
Select New POT/PO file in Poedit.
Now select the .pot file from the plugin
From the dropdown, now select the language in which you want to translate the plugin.
Click on any string you want to translate, in the bottom text input area add your translation.
After you are done translating all the strings, click save. This will give you .mo and .po files.
Upload the .mo file to your WordPress site via FTP to folder – /wp-content/languages/plugins/bb-ultimate-addon/
After you upload the file your translations will start working on your site.