As you noticed, the totals are not directly written in a template, but generated from the quote. "Bank Transaction Fee" is a custom total, added by a payment extension.
It should be a translatable string, so to change the title, use translations (yes, you can create translations for any language, even en_US).
I would suggest using theme translations, as described under "project specific theme" in What is the proper way to add theme-specific translations of new strings?
Alternatively you could use inline translations which are saved in the database:
- Add Your IP Address to the Developer Mode Whitelist (System > Configuration > Developer)
- Disable Output Caches (System > Cache Management)
- Activate the Translate Inline Tool (System > Configuration > Developer)
- Update the Text (Click the
icon next to any text in the frontend that you want to change)
- Restore Original Settings