Fiction with alien languages: In a more fantasy setting, humans teach gods to talk, creating a linguistically interesting magical system (Gretchen's thread, Bookshop, Amazon).

Fiction with alien languages: For more linguists trying to talk with aliens, check out this short story which was adapted into the movie Arrival (We talked about Arrival in this episode).

Fiction about translation: A magical version of Oxford imagines the British empire as running on magic generated from the subtle meaning differences in a translation pair of words in two languages on either side of a silver bar. (We talked about Babel and two other books with interesting word magic systems in this episode)

Fiction about translation: Another short story about the translation between the spoken word and the written word, which you can read online. 

Fiction that does interesting things with pronouns: Set on a generation starship where travel between decks is so restricted that different decks have developed different pronoun systems

Fiction that does interesting things with pronouns: Set in fantasy historical Netherlands plus a system of six genders with translation issues.

Fiction that does interesting things with pronouns: Has a human society with eight genders, each with their own pronouns (as well as aliens who don't understand them).

Fiction that does interesting things with pronouns: Has a neutral neopronoun per (short for person) in a future imagined from the 1970s that is in many ways still resonant. (neopronouns in this bonus episode with Kirby Conrod.)

Fiction that does interesting things with pronouns: Has several linguistically interesting elements, all of which are too spoilery to tell you about, but if the idea of roving adventure librarians making maps of a fantasy landscape appeals to you, we recommend it! (Gretchen's tweet, Smashwords) 

Nonfiction suitable for younger readers: Lots of short chapters about linguistics topics with vocabulary appropriate for younger readers, starting around age 10 or so. (Lauren's review)

David Crystal also has over a hundred books for grown-ups about various linguistics and language-adjacent topics, so there's plenty to choose from, though some books inevitably overlap with each other in material at this point. 

Nonfiction suitable for younger readers: Will answer a lot of your "why does English do that?" questions. It's written for adults but comes with lots of fun line drawings and short chapters which are relatively engaging for youngish readers or for parents to read and be well equipped to answer kid questions.

The Crash Course Linguistics videos are also suitable for teens, and Tom Scott's Language Files we've tested and found popular with 9 and up. 

Fiction for younger readers: The favourite YA book of both Lingthusiasm hosts independently before they met each other, this novel told through letters exchanged between residents of a fictional island nation that reveres the pan-alphabetic sentence "the quick brown fox jumps over the lazy dog" has delightful meta-fictional elements which also make it an enjoyable read for adults. (Lauren's review)