Latin (and to some extent Greek) used to be the lingua franca during the middle ages. Later on, French became the language of diplomacy and nobility. Everyone that mattered(1) speaks a local variation of said language which should still be understandable by another speaker. For example, Quebecois and French or American and English.
So, you could have such a language that all the PCs speak. They should be able to interact with everyone else. Now, make sure that each PC speaks the language from where they will go adventuring. If not, they will have to find a teacher and learn the language. This does not take that much time. You can learn everyday grammar and vocabulary in about three to six months of (hard) study. This is what I do for all my games.
Well, almost all my games. If the game is set in a bounded location, then only those languages that are around said location will be relevant. If I set a game in 14th century Venice, I do not need to worry about the PCs speaking Japanese. If I set a game in the Crusades, you better believe that everyone will learn Latin, French, and Arabic pretty damned quickly if they want to get anything done.
If you have boogly powers (aka magic or psionics or whatever), then learning languages could be done via it.
As a side note, Middle Earth started as a setting to play with the evolution of different languages yet most characters manage to communicate quiet well -- and were delayed at the gates of Moria because of a translation error!
Philology is just cool. And just because it is hard to implement in game setting should not be a barrier to trying it out provided that it enhances the enjoyment of the game.
(1) Why, yes sire, I do have blue blood... What about Peasants? They don't need to speak to outsiders, they need to work harder and pay taxes.
Evil? We're SAVING the world!
No one thinks they're the villain. Outside of a few very adolescent power fantasy type cults, nearly every other cult is based in imagining it's doing something for either the greater good or at least the good of it's members, and has rationalized all the things it has to do in that regard.
Sacrifice a baby? The baby was evil & deserved it / by killing the baby we actually set it's soul free / the world would end if we didn't / the parents have wronged us and we're stealing the baby's power, they deserve it / it's not even a real baby anyway, this is all a test by (deity), etc.
Cults run on hope, but are backed by social pressure
All cults promise something. They promise something the general society doesn't offer, and most of the time it's some form of secret power, knowledge or salvation. "The rest of the world will fall like fools because they couldn't see the TRUTH. The truth only WE know. You and I? We'll be like gods, while THEY burn! You are wise to join us!" etc.
It appeals to the desperate and secretly greedy.
Once people are in, eventually more and more commitments are requested, and the way to keep people in is the social pressure. They try to cut people off from their families or bring the whole family into the cult itself and then cut people off from social backing outside of the cult (except when, say, they're using those same connections, like a politician or celebrity). They find out secrets, they threaten and blackmail, but it's always under the guise of having to fulfill their dream, the person being "corrupted by the world", or having to protect their own.
The greater the isolation, the deeper the fanaticism goes, and the more extreme the acts of violence and abuse become because people have become normalized to it and their lines of what's reasonable are further and further astray.
Leaders are charismatic
To sell this line of hope and dreams, and to rationalize all the abuse, the leaders are charismatic. Either they're the form of salvation itself, or they're the one everyone wants to be, or be with. These leaders are very good at playing people against each other, using social bullying, rumors, and lies to keep the cult busy and obeying rather than simply fact checking with each other and realizing they're all getting played.
As much as the sexual abuse might be enacted from the leader, there's also a lot of people who will throw themselves, or offer up their partners, or even children to the leader to win favor. It's pretty much the most horrific end result of transactional thinking applied to salvation.
Layered Membership
The cult you get when you first join is not the cult you get when you're in deep. The frontward facing side of cults focused on recruiting is often no different than many other religions. There may be a few doctrinal differences, but otherwise, it's a fundraising and general recruitment pool.
Those who seem especially dogmatic or eager to throw their energy in, or, those who are willing to buy their way in and meet higher and higher cash requests, will be recruited deeper in.
It's harder to say if the fanaticism is a requirement to push the more violent, abusive and bizarre behaviors, or if, even left with relatively normal, vague commandments, fanatics will just find a way to make it violent and abusive anyway. We can see extremists of any religion take it there no matter what the original values taught were...
Taking it to fantasy
The big difference in fantasy is that religious elements are more immediately "provable". You can do magic or you can't. You can summon spirits, or you can't. This isn't a perfect set up, since, once you HAVE magic operating in your world, the reason WHY magic works and what it means theologically can be faked or rationalized a thousand ways.
A wizard with no ties to any deity or philosophy could easily pretend all their powers come from a deity and form a cult. A trickster cleric could pretend to get his powers from another god or power. etc. A ritual might always fail not because it's bullshit, but because "you haven't earned the god's favor, yet."
The other side of it is because magic is real, depending on how rare/common it is, how much people are desperate for that power or what it might promise, people are extremely motivated and possibly more vulnerable to a cult's offers.
If you have a game where magic cannot raise the dead, maybe cults form around seeking this. If you have a game where magic CAN raise the dead, maybe high end members regularly die and are brought back to "earn piety"!!!
All in all, if you're taking real world cults and putting it in your fantasy, just realize they mostly come down to money, sexual favors, and sometimes political push, such as ethnic supremacy cults. Even if the magic exists to (travel to another world, summon their god, etc.) most of the cults actually won't try to really make that happen, instead using it as a carrot to keep the cult itself wasting their energy while bleeding off on exploitation.
This also means that even jaded and well informed people will probably dismiss most cults as anything other than a minor social evil, outside of their abuse and occasional acts of public violence. So, if a cult actually DOES start tapping into real evil powers, just about everyone will be surprised by it...
(Also this means you will see violent fanatical cults supposedly dedicated to good deities as well, which sets up interesting roleplaying conflicts...)
Best Answer
In a world with mixed levels, the party should follow a simple rule:
1) Low-level adventurers should avoid the attention of high-level villains.
This rule leads to a common trend:
2) High-level villains have no interest in low-level adventurers.
High-level villains participate in the high-level world. Their enemies are other high-level people: kings, rulers, leaders of rival criminal organizations, etc. Their goals are high-level goals to match. If high-level people naturally occur in this setting, then they are natural rivals for each other.
Your example is a perfect one: the local Mafia has some pretty powerful people on its payroll, people who could take out the party if they tried. To stay alive, the party is going to have to remember rule #1. What can they do to avoid the attention of these high-level Mafia villains?