You can add pronoun support to your app, reusable across users and outputs through a simple template literal. Here's one way to do it:

pronouns.js

P = function(person, theyConjugation) {
	let conjugation = theyConjugation;

	const allPronouns = {
		"they/them": {
			"subject": "they",
			"object": "them",
			"possessive-determiner": "their",
			"possessive-pronoun": "theirs",
			"reflexive": "themselves",
			"be-present": "are",
			"be-past": "were",
			"be-present-contracted": "they're"
		},
		"she/her": {
			"subject": "she",
			"object": "her",
			"possessive-determiner": "her",
			"possessive-pronoun": "hers",
			"reflexive": "herself",
			"be-present": "is",
			"be-past": "was",
			"be-present-contracted": "she's"

		},
		"he/him": {
			"subject": "he",
			"object": "him",
			"possessive-determiner": "his",
			"possessive-pronoun": "his",
			"reflexive": "himself",
			"be-present": "is",
			"be-past": "was",
			"be-present-contracted": "he's"
		}
	};

	const isUpperCase = (conjugation.charAt(0) == conjugation.charAt(0).toUpperCase()) ? true : false;

	if (isUpperCase) { conjugation = conjugation.toLowerCase() }

	let conjugations = conjugation.split(" ");
	const reCase = (s) => { if (isUpperCase) { return s.charAt(0).toUpperCase() + s.slice(1) } else {return s } }
	const pronouns = person.pronouns ? allPronouns[person.pronouns] : allPronouns["they/them"];

	conjugations = conjugations.map(conjugation => {
		switch (conjugation) {
		  case 'they':
		    return pronouns["subject"];
		  case 'them':
		    return pronouns["object"];
		  case 'their':
		    return pronouns["possessive-determiner"];
		  case 'theirs':
		    return pronouns["possessive-pronoun"];
		  case 'themselves':
		    return pronouns["reflexive"];
		  case 'themself':
		    return pronouns["reflexive"];
		  case 'are':
		    return pronouns["be-present"];
		  case 'were':
		    return pronouns["be-past"];
		  case "they're":
		  	return pronouns["be-present-contracted"];
		  default:
		    return conjugation;
		}
	});

	return reCase(conjugations.join(" "));
}

// Usage:
// 1. Have a notion of a user or person.
// 2. Let a user select their pronouns from a dropdown provided by the keys of P.allPronouns
// 3. Use ${P(human, "They")} inside a `` string to conjugate the human's pronouns correctly!

let alex = {
	uid: 1,
	name: "Alex"
}
let carol = {
	uid: 2,
	name: "Carol"
}
let eric = {
	uid: 3,
	name: "Eric"
}

alex.pronouns = "they/them";
carol.pronouns = "she/her";
eric.pronouns = "he/him";

const getTestPronounsParagraph = (s) => `${P(s, "They")} went to the park.
 I went with ${P(s, "them")}.
 ${P(s, "They")} brought ${P(s, "their")} boomerang.
 At least I think it was ${P(s, "theirs")}.
 ${P(s, "They")} threw the boomerang to ${P(s, "themselves")}.
 ${P(s, "They were")} in the park for hours.
 I think ${P(s, "they're")} still out there!`


console.log(getTestPronounsParagraph(alex));
console.log(getTestPronounsParagraph(carol));
console.log(getTestPronounsParagraph(eric));

Outputs:

"They went to the park.
I went with them.
They brought their boomerang.
At least I think it was theirs.
They threw the boomerang to themselves.
They were in the park for hours.
I think they're still out there!"

"She went to the park.
I went with her.
She brought her boomerang.
At least I think it was hers.
She threw the boomerang to herself.
She was in the park for hours.
I think she's still out there!"

"He went to the park.
I went with him.
He brought his boomerang.
At least I think it was his.
He threw the boomerang to himself.
He was in the park for hours.
I think he's still out there!"