Players do not take the language barrier seriously enough. Maybe they have been watching too much Star Trek and expect aliens to speak English. The Fifty Worlds do not work like that. Player characters soon learn to take spells such as comprehend languages and tongues seriously.
I go into more detail below. Here is a brief overview:
The Common Tongue is the language of the City of Atlantis. It is related, albeit not all that closely, to classical Greek, and also related, albeit not all that closely, to the language of Achilles and Odysseus. Assume that player characters newly-arrived from Earth do not speak it at all. Everyone else speaks it reasonably well, but only natives of Atlantis speak it perfectly.
Most nations and cities have their own languages, but educated people will also speak the Common Tongue. People usually assume that strangers do not speak the local tongue (for the excellent reason that their assumption is usually correct); NPCs may well try to plot things in their local tongue within hearing of player characters, assuming that they will not be understood.
Basically, as long as you remember one point, you should not have much trouble. The point to remember is that puns in English do not work in other languages, but by the same token there might be puns in other languages which do not work in English. If you decide, for example, that in one city the local word for dragon sounds very much like the local word for fertiliser, that's fine, and could well lead to some interesting misunderstandings. It also allows you free rein to misinterpret wishes however you like - if you feel that a player character is asking for something over the top, something he said has a second meaning in the particular language he made the wish in.
There is one more point: swear-words are the names of demons. The stronger the swear-word, the more powerful the demon (really strong swear-words are the names of balors or mariliths). And demons often turn up whenever they hear their names spoken aloud ...
In most languages, specifically including the Common Tongue, verbs and nouns have different word-endings. It is entirely acceptable to use any verb as a command even if, in English, the same word could also be some other part of speech, such as a noun.
If you think that your players are up to spotting subtle linguistic clues, here are a few possibilities. They are the sort of thing a real spy would notice.
In Xenby itself, the X represents a sneezing sound, and the y is pronounced as ae in Latin. So Xenby is pronounced Chen-bae, with the stress on the first syllable.
In Atlantean, and the Common Tongue, X at the beginning of a word is pronounced Gz (as it is in English). N in front of b, m or p is pronounced as m (as it is in Japanese). Y is pronounced rather like Greek upsilon: a German u-umlaut in proper Atlantean and i in the Common Tongue. There are no stressed syllables. So an Atlantean would pronounce Xenby as Gzembue and believe that this pronunciation was correct.
So you're in Omnatia, a city notorious for spies, and someone walks up to you and says he's a wizard from Xenby, pronouncing it Gzembi. Do you believe him?
There's a young lady named Bda. I don't think she's in phase one, but she might turn up sooner or later.
In Atlantean, at the beginnings of words, X, Kt, Ps and Pt are pronounced Gz, Gd, Bz and Bd respectively.
Words in the Common Tongue also start with these combinations of consonants, but they are spelt the way they sound, gamma-zeta, gamma-delta, beta-zeta and beta-delta respectively.
So you're taking a stagecoach, and one of the other passengers is a young lady dressed as an Atlantean aristocrat. You see the coachman loading her trunk. It has an Atlantean coat of arms with a beta monogram next to it. You ask the young lady what the beta stands for, and she says it's her name, Bda. Do you believe her?
Natives of Atlantis all speak a language which they call Atlantean. They speak it very well, and very quickly. It is vaguely related to classical Greek.
Atlantis is both a seaport and a huge city with large immigrant communities. The Common Tongue is a simplified version of Atlantean, a commercial patois spoken by non-native residents (including recent immigrants as well as retired adventurers) and by anyone in the shipping industry. Native Atlanteans can understand the Common Tongue (it sounds to them like bad Atlantean) but sometimes find it difficult to express themselves clearly using it.
Aristocratic Atlanteans often look down their aristocratic noses at anyone speaking the Common Tongue, but everyone else in Atlantis is happy enough to use it if there is a profit in it somewhere.
Natives of the countryside to the east of Atlantis speak languages of their own which, while clearly related to Atlantean, are not mutually comprehensible with it. There are no distinct languages, just gradual shifts as one travels further from Atlantis. As a general rule, peasants living a hundred miles apart will only have a 50% chance of understanding one another when speaking their native languages. Tradesmen will usually also speak the Common Tongue.
Natives of the city of Xenby speak their own language, a hybrid of Atlantean and the language of the countryside around Xenby. It sounds similar enough to standard Atlantean, but there are enough differences to make serious misunderstandings likely. Native Xenbians can understand the Common Tongue (it sounds to them like bad Xenbian) but sometimes find it difficult to express themselves clearly using it.
Natives of the city of Thoqmaxar speak their own language, Thoqmaxi, which is vaguely related to ancient Persian. Tradesmen also speak the Desert Tongue, a trading language which sounds like a simplified version of Thoqmaxi. Educated residents also speak the Common Tongue.
The city of Omnatia is cosmopolitan. Most natives speak Thoqmaxi, and everyone speaks the Desert Tongue. There is a large Odinist community which speaks a language vaguely related to Icelandic; educated residents from outside the Odinist community often speak this language too. There are many visitors from Atlantis, and not a few have settled in the city. They speak Atlantean, and have the same problems with the Common Tongue as other Atlanteans. Everyone else, except for some particularly lazy Thoqmaxi-speakers, speaks the Common Tongue.
The oasis of Agvhazar does not have its own language. The official language is the Desert Tongue. Most people will speak at least one other language, which might be Thoqmaxi, Havdazi, the Odinist language or the Common Tongue.
The official language of the city of Havdazar is Havdazi, which is vaguely related to ancient Persian. It is clearly of the same family as Thoqmaxi, but the two languages are not mutually comprehensible. The authorities frown on the use of any other language. However, since the army employs a large number of Atlantean mercenaries (who speak Atlantean to one another and have a better-than-usual ability at communicating in the Common Tongue), the Common Tongue is often used within the army. Tradesmen are usually able to speak the Desert Tongue, but may not be willing to admit this.
The official language of the city of Ligaea is Ligaean: the party line is that this was the language of the Trojans. It is vaguely related to Etruscan. Several other languages are spoken in the city, including not only the Common Tongue but Sea Elven, various languages from cities and islands to the west of the Ligaean Peninsula, and the trading language of the sea (which is vaguely Germanic). Anyone likely to come into contact with foreigners is likely to speak at least one other language in addition to Ligaean (but not necessarily the right one for any particular group of foreigners).
The official language of the city of Rajeya is Rajeyan, which is vaguely Italianate. Rajeyan and Ligaean are clearly from the same family, but are not mutually-comprehensible. Several other languages are spoken in the city, including not only the Common Tongue but various languages from cities to the east of the Ligaean Peninsula, and the trading language of the sea (which is vaguely Germanic). On the whole, Rajeyans like learning languages, and most of them will speak the Common Tongue and one or two others in addition to their native tongue.
The language of the city of Bianzie is Wregnish, which is not similar to any Earth language. Although various other cities, similar in size to Bianzie, also speak Wregnish, it is the Bianzie dialect which is deemed to be standard Wregnish. The various dialects of Wregnish are clearly distinct, but are mutually-comprehensible - the chance of a serious misunderstanding is no more than 10%. Educated people also speak the Common Tongue.
The residents of the city of Vlydyga also speak a dialect of Wregnish as their first language. It has diverged somewhat over the years, and the chance of a serious misunderstanding between a speaker of the Vlydygan dialect and a speaker of standard Wregnish is around 25%. Educated people also speak the Common Tongue. The people of the surrounding countryside speak a completely different language, unrelated to Wregnish; there is little contact between city-dwellers and countryside-dwellers.
The language of the city of Qwanet is Qwanetian, which is not similar to any Earth language (however, the Qwanetian language is written in Thaana script, used on Earth for Divehi). Some residents speak other languages, the Common Tongue being one of the more common as travel between Qwanet and Atlantis is not unknown.
Natives of the hinterland behind Qwanet speak languages of their own which, while clearly related to Qwanetian (and which also use Thaana script), are not mutually comprehensible with it. There are no distinct languages, just gradual shifts as one travels further from Qwanet. As a general rule, peasants living a hundred miles apart will only have a 50% chance of understanding one another when speaking their native languages. Tradesmen will sometimes also speak the Common Tongue.
Residents of the Yjsbson Empire speak Yjsbson. It is a frighteningly difficult language, and not similar to any Earth language. Even native speakers do not speak it all that well. Hardly anyone from any other world speaks the language at all. Educated people often speak a second language. In the north of the Empire, this might be Qwanetian or the Common Tongue. In the south of the Empire, it is more likely to be another, equally unpronounceable, indigenous language.
Residents of the few civilised places on Salta all speak the same language, Saltese. It is not similar to any Earth language. Saltese priests refuse to speak any other language. Other educated people will usually speak one or two other languages, which might or might not be the Common Tongue or the Vlydygan dialect of Wregnish.
The major cities on Regibry each have their own language. The languages are related but mutual comprehension is difficult. They are vaguely related to ancient Egyptian. Offworld visitors are common, so most tradesmen also speak the Common Tongue. Outside the cities, peasants only speak the local language but the educated will also speak their Imperial language and possibly also the Common Tongue.
The city of Megalopolis was founded by adventurers from various cities. The official language, as spoken by the Knights, is a simplified form of Atlantean which is similar to, but not quite the same as, the Common Tongue (the chances of a serious misunderstanding are about 10%). It is written, not in Greek alphabet, but in the Imperial Script of Darius. Many knights also speak other languages, Rajeyan being one of many.
The most commonly-spoken language in Megalopolis is the Desert Tongue, the trading language used in cities such as Thoqmaxar, Agvhazar and Havdazar. It is the language of the tradesmen and merchants who turned up in the city after the Knights had cleared the area of monsters.
Descendants of the indigenous tribesmen who lived in the area before the Knights turned up will also speak their tribal language. These are unrelated to any Earth language, and the various tribal languages are only partially mutually-comprehensible. There is no chance of a misunderstanding - people whose ancestors were from different tribes know that their tribal languages are not the same, and speak to one another either in the Megalopolitan version of the Common Tongue or in the Desert Tongue (everyone speaks one or the other, and some people speak both).
The natives of Tsing speak their own language. It is not particularly hard to learn. However, their thought-processes are incomprehensible to anyone from other worlds, so even having learnt their language is is still impossible to communicate sensibly with them.
Different types of elves speak different languages. None of these resembles Quenya in any way whatsoever. I gather that although neither elves in themselves, nor Quenya in itself, is copyright, elves who speak Quenya are. So elves in the Fifty Worlds don't. Elves do not usually speak human languages - they expect humans to learn theirs, and sneer when the humans do not speak it all that well. Elves are not nice people.
Dwarves speak Glaswegian, orcs speak Brummie, and hobbits speak Cockney. There is little point in learning any of them, as none of them has anything all that interesting to say. Dwarves, orcs and hobbits are not nice people either.
According to the Jesuits (who ought to know), devils speak Japanese. Who am I to disagree? So, in the Fifty Worlds, devils speak Japanese.
Demons speak Buenos Aires slang, partly because Buenos Aires is the furthest place on Earth from Japan, and partly because a well-known opera set in Buenos Aires has a demon as its lead character.