Crusader Kings III

I recommend doing this achievement alongside a longer regular playthrough, not to start a playthrough with a custom character specifically to get this, because it takes some time and it can easily be done casually on the side while doing other stuff if you have the right type of ruler character for it. Also it is much easier to prepare your player heir to become polyglot than to do it as an adult starting character.

We need a character with a high learning skill, ideally one with a learning education trait and the trait "genius" or at least "intelligent". It can also be done by a character with a different education than learning, as long as you have a high enough learning skill and spend a few of your character's life years in the learning life style. If you are going for an efficient long term playthrough, you will usually try to "breed" your player heirs to be genius anyway and learning education is generally a good choice for midgame, because it's the best way to get characters that are good at everything (especially thanks to the "learn on the job" perk) and have a long life expectancy. So once you have an intelligent or genius child with the curious or pensive trait, it's time to train them as a your heir to become a great philosopher king and get that little language achievement on the side.

If your spouse is from another culture, your children are likely to pick up their language automatically by observation. The same works with a guardian of a different culture, if you don't want to educate your player heir personally. Once the child is six years old, use the "have child study language" interaction on them (requires employing a court tutor). By doing those things, your child will hopefully already know 3-5 languages when they come of age.

The most challenging part of the achievement is raising your foreign language limit up to 9 (You can go over the limit, but learning new languages will get harder and you'll get frequent random events that force you to either forget a language or take a lot of stress). Your foreign language limit is base 1 plus 1 more for every 5 points in learning. The perk "open-minded" from the scholar skill tree gives you 2 more, so we need to get to 30 points in learning to get to 9 foreign languages.
(In theory, you could also get the dynasty legacy "language scholars" for one more language and/or the perk "smooth operator" from the seduction skill tree for one more language, but I'd recommend neither of those, since the seduction skill tree is borderline useless most of the time and the dynasty legacy path that starts with language scholars is also quite underwhelming, so don't waste your precious dynasty fame on that).

To get your heir's learning skill to 30 eventually, first pick a good wife with a high learning skill for him, so she can give him a lot of bonus learning points via the council task. You'll probably want to marry your genius heir to one of their genius cousins to produce more genius dynasty members anyway, so finding one with a good learning skill shouldn't be too difficult at that point ;.). If that's still not enough to get to 30, the scholar skill tree has the perk "scholarly circles" that gives you plus 1 learning for every level of devotion and the perk "learn on the job" gives you a bonus of 20% of your councillors' primary skills to all of your skills, so you can get 3 to 5 points more from a skilled realm priest (It helps if you have a religion that let's you hire and fire your realm priests, so you don't need to keep murdering your bishops until the pope finally sends you one that's actually good at his job). You can also do a few pilgrim journeys to max out your pilgrim and traveller experience traits for another 5 learning total. With all that, it should be no problem to get to learning 30 and beyond, even if you use a character who doesn't have the learning education trait and only a mediocre learning stat to start.

So after your heir reaches adulthood, there's not much more you can do to make him learn more languages until your current character dies and the heir takes over. Keep in mind that learning all those languages will take a few years (about two per language), so maybe don't try to celebrate your 100th birthday with your current character, but start taking some stress and some risks before your heir gets to old. Make sure to give your heir some safe land to rule as your vassal, so he starts building up his fame and earning perks in his lifestyle.

Once your heir takes over, you'll probably first need to sway some unruly vassals and don't want to start learning languages right away. I recommend starting your reign with a diplomacy lifestyle to get the perks "thoughtful" and "adaptive traditions". The latter one allows you to do two learn language schemes simultaneously or do another personal scheme alongside a learn language scheme, so you can still sway or romance characters while you are busy learning. After 5 years, switch back to the learning life style to pick up those traits from the scholar tree, if your character doesn't have them yet.

Now all that's left to do is learn the remaining languages. With your high learning and the pedagogy perk, your success chance should be at the max of 95 %. To learn a language, you have to start a personal "learn language" scheme against a character who speaks that language. You can pick any character within your diplomatic range, they don't have to be at your court. Their values and their attitude don't matter, they won't "resist" your learning scheme, although target character who do like you (especially spouses, lovers, friends) might trigger random events in which they help you progress in your studies.

Keep in mind that your learning scheme is aborted and all progress lost if your target character dies in those about 2 years it takes to complete the scheme, so best avoid targetting characters that are very old, sick, currently rebelling against their liege or fighting in war as knights.