Effulgence and other glowfic has characters who speak a whole hell of a lot of languages, and its authors are only fluent in English, and its readers are only expected to be fluent in English. Here are some things that come to mind about how we're dealing with that. Feel free to use this thread to ask questions about other translation convention minutiae.
1. In Arceus, we not only refer to Pokémon species by their often-punny English names, we render the Pokémon's own vocalizations as those names (remixed accordingly). In Arceus, they don't speak English and there is no reason for their fauna to have names made of English phonemes, let alone puns. The translation convention is thus: because of our slightly canon-puncturing approach to Arceus (the Pokémon franchise still exists in Earths, but most of the game- and anime- canon we're using for Arceus proper gets the "equivalent but not particularly reminiscent" treatment) we're just translating the species names. But, in Arceus languages, 'mon species have names closely related to their vocalizations in most cases. Rather than make up separate vocalizations and fake-etymological names for each kind of 'mon we mention, we're translating the sounds in both cases to the readily available English terms. The "actual sounds" that 'mon and characters are emitting are not those we write down, but this is easier to write and follow along with. When Arceus peals, some non-Earthling will be confused that their English pentagon supplies translations for the names of these unfamiliar creatures and somebody from a modern Earth will go "UM I can explain that, but it will just prompt more questions."
2. See the Name Attractors thread for a description of how I'm handling Bell surnames, which are oriented around meaning.
3. In Equestria, they do speak literal English. There was no good way to get around that given how the show's vocabulary is constructed and how I wanted to name Clarity. They certainly have accent and dialect differences from the peal's commonest form of English, but the ponies are more mutually intelligible with that form than, say, Cymbeline's Loegrian. Glass will produce vague metacausal mutterings about this but the characters will mostly be pretty confused.
4. Far-future characters who speak something labeled in their canon or our worldbuilding as "English" will find that they can still hold comprehensible conversations with everyone else in the peal-and-its-social-environs who also has that conversation, even without the benefit of coins/the Speech/whatever. This in spite of the fact that Linyabel was born in 2977 and on Eta Ceta, while Aegis, Tab, Shell Bell, T'Mir, and (if we ever get around to her) Lailah are all from different nearer-future branches of Earth's forays into the future and Linya trying to talk to any Standard Backstory Bell without translation help should be like someone from modern-day Singapore trying to communicate with the author of Beowulf only more so and the other five's native tongues shouldn't be that much more similar to each other than any pair of Romance languages. This is because a lot of narrative dwelling on linguistics would be boring and distract from the main point. They also tend to meet in Milliways, which has plenty of ambient translation magic.
5. On Earthalikes (Thalassa, Lake, Rêverie, Treasury, and some unthreaded worlds such as Summer's unnamed one) there will tend to be analogues to major Earth countries and languages, with different names that I get by poking the Etymology section of relevant Wikipedia articles until they scream and keep in a spreadsheet so I don't accidentally refer to Callia in Lake or Diotaland in Treasury or whatever. Cymbeline is the only Earthalike-dwelling Bell whose native language is the English-analogue (Loegrian); Rêverie is the only other world where I've even bothered to name the English-analogue (Albish). Unaided speakers of analogue languages should be able to muddle along about as well as speakers of German and Yiddish - which is to say, some, but you have to really be trying. Whenever I want to produce a non-place-name word in an Earthalike analogue-language, though, I tend to just directly (if naively) translate into the Earth equivalent ("cygne", for instance).
6. I try to avoid making puns that don't seem like they'd translate, when I write characters who are not that moment speaking English. I try to encourage my coauthors likewise. There are probably missed puns here and there. Alas.
7. Some of the languages of Materia, while not Earthalike-analogue languages, have a certain metacausal sympathy with certain Earth languages. This makes negligible practical difference to intelligibility (there is none) but idioms and so on will tend to match more closely between Pax and English than between, say, Marlese and English. This metacausal sympathy is there because Alexandra Erin, author of the Tales of MU canon from which Materia is drawn, has a consistent habit of rendering some of her setting's languages as real languages (French, Latin, etc.) and is much less inclined to restraining herself from puns while under translation convention than I am.
8. The prevailing vernacular from Chi's world (which may be renamed as I catch up to Korra and can evaluate Kappa's suggested name for it) is not Chinese, an analogue of Chinese, rendered in Chinese characters, mutually intelligible to speakers of any Chinese dialect, or possessed of any metacauasal sympathy to Chinese like the Pax/English one. However, when I need a word from that language, I grab a Chinese one (naively translated to Mandarin, rendered without tones in pinyin), because I took a year and a half of Chinese in college and have the first idea of how to do that rather than making up probably-vaguely-racist wholesale inventions. If I need to do the same thing with a second language from the same world, I will probably use Japanese the same way (because I used to watch a whole lot of subtitled anime). Chi's roc Liqing is named the Chinese name I was assigned when I took the class. Similar things are going on in the background with Old Norse and Edda (plus Yggdrasil by extension) even though I don't know any Old Norse but what Wikipedia tells me and even though there is a literal culture literally speaking Old Norse on a real not even -alike Earth in both of the MCU-y worlds.
9. When canon provides me with a dialect's slang, I use it, and the other characters hear it without the benefit of translation convention. If Aegis starts slinging Battle School rude words, then that's what she's saying, and being in the 'speaks English' category will not supply details to eavesdroppers. It's more fun that way.
10. Narnians speak English with a Britishesque accent. If you read the Magician's Nephew this makes sense. Something functionally equivalent will be afoot in Oz if we ever get to Oz, but not exactly that thing.