If there are any footer boxes (modules) installed and enabled, find them and add them to the footer code. Each box (module) will have it's own language file. That's why there is no 'boxes_footer' in the language files.
There are multiple 'boxes' available to install into the footer. In your Admin, click on 'Modules' on the left hand side. Then click on 'Install Modules' in the upper right corner. You will find additional 'boxes' or modules you can install.
Each module has it's own language file, usually in catalog/admin/includes/languages/<your language (typically English)>/<module_name>.php
It's in the language file that you change the text.