"My experience from France was nice."
Translation:Η εμπειρία μου από τη Γαλλία ήταν ωραία.
Nice can be translated as καλός, ωραίος, ευγενικός with many other meanings hiding in it depending on context, e.g. pleasant, beautiful etc. In this case, the best translation is ωραία, simply because μία καλή εμπειρία is a good experience. So, it's not that καλή does not work in this sentence at all, it's just that there is a more accurate way of translating this phrase with better word matching. I suggest reading the examples here and here. In short, καλός=good but this is usually too restrictive.