You have two solutions:
1. Find, in PayPal, the button image that you want. You can find it during button creation. Right clcik on it, save it on your computer, and then upload it on your site. Start over the procedure of creating the button. At this point, PayPal asks if you want to use a button image of your own. Give it the URL of the imafe that you just uploaded in your site, and you are done.
2. Usually, the paypal button code finishes wit a code like this:
<input type="image" src="https://www.paypal.com/
fr_FR/i/btn/x-click-but02.gif" border="0" name="submit" alt="Effettua i tuoi pagamenti con PayPal. un sistema rapido, gratuito e sicuro.">
<img alt="" border="0" src="https://www.paypal.com/it_IT/i/scr/pixel.gif" width="1" height="1">
</form>
The part in red is the button image source path. As you see, it must be
fr_FR to retrieve the French image.
You can manually change this in the code that you are copying / pasting from PayPal.
So, create the button code in PayPal. When done, copy the code PayPal gives you. Drag a HTML box in your BV page. Make it approx the same size as the button you have chosen. Double click it and paste there the code that you have copied frpm PayPAl. If it still gives you an English button, change the code as described.
Last, if you wish, provide a link to your page, and i can then fix the code for you (but only if it is a NON encrypted button code).