He beams right back, asking her what needs to be done for it to happen. The spell Twilight knew was an enchantment: she could embed her own understanding of Ponish into an item that can be worn, like a ring or an amulet. And when somepony does wear it, it interprets the meaning of whatever they say, and plays a translation out loud. It wasn’t perfect, and it certainly wasn’t a real-time method, but it was also a stepping stone to a more potent version of the enchantment, which contained a complete understanding of both languages and could translate perfectly between the two in real time. That version was used by unicorn historians to preserve old languages, but she sadly didn’t remember the details out of hoof: she would need to ask for a copy of the spell.