You'll need to create a language file for the plugin to represent each language you want to translate.
Basically, in your administrator folder, you'll find administrator/language/en-GB/en-GB.plg_system_topofthepage.ini
Copy that file into your needed language folders, so copy it to administrator/language/xx-XX/xx-XX.plg_system_topofthepage.ini
Then edit the file, changing the value within quotes to the appropriate word or phrase in your target language. The specific line you'll need to translate is:
PLG_SYS_TOPOFTHEPAGE_GOTOTOP="Return to Top"
The rest of the file is for the plugin translations in the backend.