Ideally you should set your text editor to use iso-8859-1 or iso-8859-2 encoding first, then enter your text, including diacritics as normal, then use the nBill installer to install the language pack - this will convert the encoding to UTF-8, preserving the diacritics. Alternatively, you could keep everything in UTF-8 and just overwrite the files directly without using the nBill installer (they will have to be converted back to iso-8859-1 before they can be installed with the installer, but I can do that if necessary)
Please be aware however, that mariodantas is also working on completing the translation into French, so your efforts may be duplicated!