Individual notifications are designed to only be for one specific language. If you have 100 English notifications and want to add Portuguese support, you'll need to either duplicate all 100 of those notifications (and add conditions like Abdul mentioned) or add scripting logic. Scripting as a solution likely won't work if your notification has more than one recipient (one recipient might want Portuguese while the other English).
The "Preview Notification" button on a Notification is really helpful for this as it can tell you what users will be included or not without actually triggering the notification. You'll be able to see if your Portuguese notifications would be sent to an English user. Play around with conditions until the English users are no longer getting the notifications.
This community post is pretty useful for describing what to do with duplicating notifications:
Multi-lingual Email notifications/templates
You can also search google for a scripting approach. I came across a couple that seemed like they would likely work.
