In your class, you need to import amount_to_text_en for amount in word in english and amount_to_text_fr for amount in word in french, like this:

from openerp.tools.amount_to_text import amount_to_text_en

For more information about these tools, see the link below:

http://members.hellug.gr/xrg/openerp-doc/html/api_reference/tools/amount_to_text.html