Hi Oscar,
There are 2 flavors of the sendNotification method, one takes IapiAccountIfc for recipient and the other takes a String array for recipients. Looks like you are using the one that takes a String array by passing a list of recipients email id. I'd suggest using the first one by passing the IapiAccountIfc for the recipient and the email sent out will be in the default language of that user. The default language for the user is set on the user account object.
Hope this helps.
Regards,
Vikram