Go to your phpmyadmin and look to see how the charecters look like there. Is your database charecterset in whmcs same as blesta ? look in both whmcs and blesta in phpmyadmin and check to see. I think Blesta guys are not experienced with different languages and mostly work in English but hopefully in future things will change
We use it for invoicing our retail customers and recurring invoices for our managed services clients.
Works very well. We export the transactions into QuickBooks using some PHP code posted in the Blesta forums.