On my test site I used the Euro currency symbol as my currency to test this and it seems to render fine on all browsers. Can you please copy and paste the following character in the "Currency Symbol" field and let me know if it makes a difference?
€
With the qunaity field I think I know what the problem is. You need to make sure to save the language file in UTF-8 after you edit it. All the language files are in UTF8 but if you edit it and save it using an ordinary text editor it will get converted to ASCI and then you will start to have problems like this. You can use PSPAD text editor (it's free) that lets you save/format the text file in UTF8.
I will send you an updated copy of the plugin. Once you get the plugin do not customize the language just choose Italian from the language dropdown and you will see that the quantity field won't have an accent problem anymore. Once you understand this then go ahead and start customizing the language file but making sure when you upload it the file is in UTF8 format (simply open the file in pspad editor and check under the "format" menu).