So I’ve done a bit of conlanging for some of my non-IF fantasy stories. And I also assisted an indie game dev with the languages in his game, primarily with the phonology. I’m also currently using Old English in the draft of a story, though I may change that to a conlang for the final draft. And I was a linguistics minor (the major required taking European studies classes and I didn’t have room for those) in undergrad. So I’ve definitely gone down the rabbit hole…
Using a real language (whether or not it is still natively spoken) carries risks. People can easily do research to figure out if you’re correctly using a language and native speakers will be put off by any errors that show up. If you go this route you will want to get a native speaker (for living languages) or expert (for dead languages) to look over it if you aren’t one of those for the language you use.
Please stay away from random gibberish. It is immersion breaking to discover that what you thought was a conlang is actually random sounds, especially if there isn’t any consistency in the phonology. If you are taking the time to include gibberish, why not spend just a bit more time to develop the phonology? It will make people like me much happier and more immersed in your world.
James Portnow points out in his chapter in From Elvish to Klingon: Exploring Invented Languages (2011) that:
designers must create languages that reward the efforts of novice players, are learnable in the context of their games (there are no textbooks or teachers), are inessential to game play, fit the creative property, and can be learnt at unknown intervals. If a game doesn’t do all of these things, it doesn’t promote players’ engagement or improve players’ experience within the fiction of the game, both outcomes which should be a designer’s constant goals. (pp. 137-8)
Since interactive fiction is often a combination of prose story and game, the above points are worth keeping in mind. Your readers shouldn’t have to learn the additional language(s) that show up in your work to be able to progress. Anything important in a language other than the one the work is primarily written in should be translated. If it isn’t important, but the PC knows the language, then it should be translated. If it isn’t important and the PC doesn’t know the language, then you can leave it untranslated with a mention that the PC doesn’t know what was said. Granted, it might be good to then have the speaker switch to a language the PC does know and repeat the information. I might enjoy the puzzle of figuring out the meaning of an untranslated bit of dialogue, but someone else won’t.
I think that @TSSL’s suggestion of creating enough of a language for names is a great one. It allows you to flesh out the world a bit without having to worry too much about grammar (though you’ll probably want to figure out some of the morphology). Be careful in the use of apostrophes for conveying phonological information–they have been used for glottal stops, syllable markers, ejectives, elision, and palatalisation to name a few. If I see an apostrophe in what appears to be a conlange and there isn’t a pronunciation guide, I have no idea how I’m supposed to pronounce the word.
And speaking of Tolkien and The Lord of the Rings–in the prologue Tolkien writes about the Red Book of Westmarch and how a selection from it was published as The Hobbit, in Appendix E, Tolkien writes that “The Westron or Common Speech has been entirely translated into English equivalents,” and in Appendix F, [section] II: “On Translation” he goes further and explains how he translated the Red Book. In other words, Tolkien explains the use of English as the common language in Middle Earth as his doing as translator of The Red Book of Westmarch. Such a narrative frame is one way you could explain the fact that in your fantasy world everyone seems to be speaking English. For instance, you could say something along the lines of, “These are the collected writings of the deeds of a figure of note in [time period name] now translated from [conlang name] into English.”
In addition to TSSL’s suggestion of The Language Construction Kit, I would recommend The Art of Language Invention: From Horse-Lords to Dark Elves, the Words Behind World-Building by David J. Peterson, the linguist who created the spoken Dothraki in the Game of Thrones show based on the Dothraki words appearing in the books.
And like TSSL, I can also offer advice and suggestions on conlanging.