What did you do on your site and you wouldn't repeat it if you turned back time.
I mean things that continue. E.g. you used an important plugin that doesn't suit you but is critical and you need to keep using it. And these types of things that continue to this day. I don't know e.g. you used an elementor and you would prefer another one, you used a domain other than you wanted and you don't want to redirect etc.
In my case there are rather no such things, I rethought the architecture of the site earlier. However, if I had to choose, I don't know if not the number of languages. I've always wanted to have a multilingual site and chose software for which the API doesn't match (but I would choose it again anyway because it's much better than the competition) and everything has to be done manually copy-paste (something for something, so that a page in a foreign language can be completely different) However, it consumes some time and is tiring, even though there are only 3 languages.