Yes, I'm afraid this is a bit of a problem - when I add new features to nBill, I cannot wait for translators (who are all volunteers) to translate all the new bits before release. So new items remain in English until such time as someone kindly updates the language pack (I cannot afford to pay professional translators in all those languages).
In this case, the text for the different payment frequencies (weekly, monthly, etc.) was translated in an earlier version (as it appears elsewhere in the component), but the text for the order summary page was added later and the text is built programatically depending on the payment frequency used rather than having to store what is basically the same sentence repeatedly for each payment frequency - hence the mixture of English and French in the same sentence.
If you want to change/translate the text yourself, you will find it in the following places (replace en-GB with your language, and the line numbers might be slightly out, but you should be able to find your way):
/administrator/components/com_nbill/language/en-GB/frontend.en-GB.php (lines 122 to 126 ish)
/administrator/components/com_nbill/language/en-GB/xref.en-GB.php (lines 67 to 77 ish)