It's worth noting that notifications are not individualized per user. Let's say you have a notification where you send the notification to every member of a group. Only one email will be generated and it will be sent to all members of that group. Regardless if one member prefers English and one prefers Spanish, the notification will go out in only one language. Even if the notification has only one recipient, there is nothing to determine that user's language and all users should receive the same notifications in the same language even if that notification doesn't match their preferred language.
The workarounds currently are painful. Imagine you have a notification that goes out in English. You could duplicate that notification and create a Spanish form of the notification. Then in an email script attached to the notification you could remove recipients from the English version that prefer a non-English language. You would do this for each notification and each language you'd like to support. Clearly, this isn't the ideal approach, but there isn't a particularly good way to accomplish this currently.
