This is my personal opinion. I think it is inadvisable for someone to say prayers in Latin when they truly have no idea at all what they are saying. Such would be the case if, for example, you didn’t even know if the prayer was the Our Father, or the Apostles Creed, etc.
On the other hand, if you know that you’re praying the Our Father (for example) and you know the Our Father in English, then I don’t see any problem with saying the Our Father in Latin, even if you don’t understand every word or know exactly where you are in the prayer. In this case, you know what you are praying.
Of course, it is even better if you fully understand the Latin prayer. This can be the case even if you don’t really know Latin. You can memorize a given prayer in Latin (Our Father, for example) and look up and memorize the meanings of all of the words such that you do, in fact, know exactly, word for word, what you are praying in Latin even if you don’t know Latin generally.