In light of the recent update, (and passive aggressive YouTube comments) I'm interested in creating a mod to customize pronouns, and possibly titles, for the sims. Sims are assigned a pronoun based on gender, and I'd like to add to that with the singular pronoun They.
For example, "Robert just finished their homework."
Or, if possible to change titles, have the option of what a sim would be, for example, husband, wife, or spouse.

The idea of this mod would be to open up the options of sims, and to be more inclusive.
I'm coming to forums to find out what kind of mod I need to make. Would this be considered a script mod or override or what?
I'd really like to try to make something like this, though I may be in over my head.
I appreciate any and all input.
Thanks

Two ways to do it that I can think of, both of which are an ass-ton of work. The elegant way is to add a new gender option, and then edit every string (yes, EVERY STRING) that references sim genders to use the appropriate pronoun. The inelegant way is just to replace all the existing gendered pronouns with non-gendered pronouns.

The first way may require some script modding--probably not a huge amount, but a little (and more than I know how to do). Both ways require LOTS of xml modding (which is usually easy). There might be a way to do some kind of massive find/replace, but pronouns are built into the game string by string, so the obvious/easy way to edit them is the same (unfortunately). Replacing pronouns in a few key places that show up a lot would be relatively easy (you just have to find those files). Doing it in anything approaching a complete way would be a ton of work... and liable to create conflicts with other mods/break when the game gets updated.

Sorry, I'm sure this is not the answer you wanted, and it would be lovely to be able to have sims with non-binary pronouns. It's possible someone could come up with a better way that simply overwrites pronouns when certain triggers are met, but I suspect it would take some fancier coding if it is doable. You need an answer from someone who is better at this than I am, but I've done just enough modding to have a fairly good sense of what is and isn't achievable with the simpler modding methods.

I know this thread is more than two years old now, but did you ever start working on this? If not, I'm probably going to at some point because (as a nonbinary person myself) it's pretty important for me.
I think I'll attempt the elegant way first and we'll see how that goes; I'm newer to working on the Sims 4, but I've made a few mods for previous games over the years. If I can work it out, I may try and make a nonbinary (and potentially an intersex) option for CAS, and have it be the trigger for neutral pronouns, or have a "gender menu" you can access ingame to let you select pronouns and terms individually on a per-sim basis. Either way, it's probably doable, just a lot of work that I don't blame anyone for not wanting to do, honestly. All of this should have been implemented as a part of the game.

From the quite extensive knowledge I have, although not as fresh as it was I while ago I did much modding for the sims, I would like to give a more informed view of the situation.

Yes, there is several solutions to this. The three that comes to mind right now is 1) The very hard solution, 2) The hard solution, 3) The easy solution. I know that solution 3 is doable, I have a suspicion that 1 and 2 might not be possible.

Solution 1: Add a third gender, I will not go into extensive details about this, but it's safe to say that this would be A LOT OF WORK, and you will probably reach the limits of modding quite quickly. The run down is that gender is very strongly coded into the core game and the new gender options are tacked on on top of that. That is probably the reason why that didn't change the gender binary with the gender option update.

Solution 2: Add a gender pronoun option, this would be similar to the additional gender options. This might be more doable than 1, and would be not as much work, still A Lot Of Work. And again you will probably run in to limitations of what can be modded. I don't know how they select the strings, but as far as I know they do it in the C++ part of the game, and that is not moddable*. It could be feasible, but my assessment is that it is not.

Solution 3: Change all strings to use non-binary pronouns. This is very doable and is actually not that hard, it is just a lot of work. The way the game stores names is in one big file for the base game and one for each pack. Here the big issues is that as far as I know there aren't any tools that let you automate the process of editing the string tables yet, so you will have to manually edit each string. Pronouns are easy they are written as "{M0.He}{F0.She}" and could just be replaced with "They". The titles such as "wife" and "husband" would have to be manually found and both replaced with "spouse", that part would need to be done manually anyway. What you need to do is to is open up the games string tables, find and copy each string that need to be changed into a new string table and edit them there. This is so that you don't have to redo your work for each game update, you just need to find the new strings and update them.

To summarise:
Your only feasible option is option 3, but it is a lot of work. It would be nice to automate this, but there isn't any tools for that yet, as far as I know. But by thinking through your process thoroughly and looking up what tools you have available you can definitely do this, and be able to update with game patches.

That being said, I am a toolmaker and I love automation. I am also strongly in favour of making things more inclusive. Even though I am not very interested in this mod personally, I am actually interested in making one now. However, I am quite busy so not sure when that would be. So you should probably get on it if you want to guarantee that it gets made.

The files containing the strings are named Strings_ENG_US.package (for American english) and the files containing the strings are called STBL or string tables.

Edit:
Just realised that s4pe or sims4studio might have the option to import and export to cvs or other similar formats, that would allow you to search and replace with a text editor or a spreadsheet program.

Like Fogity I'm not sure if either of the main editors will import/export STBL in any format other than a binary file, but this program should still work (I've used it relatively recently): http://modthesims.info/t/533109

So, as I said, I got intrigued about this and have been working on my tools. I can now comfortable say that I will be releasing a netral gender mod in the near future.
I mainly just need to fix up some little extra peices of code, figure out what the gender neutral version of some phrases are, and figure out some rules I can use to fix poor grammar as a result of my substitutions.
Everything else is ready, I tried a test version of the mod in the game today.

For completeness I want to mention that I have finished my mod and it is available on my site.

Just out of curiosity, is there a way to decide which sims use gender-neutral pronouns with this? Like, so that some sims can use she/her or he/him, while others use they/them?
I'm just asking because in my experience so far it uses they/them for all my sims, regardless of gender.
Also, I just wanna add; thank you so much for making this mod. It may not seem like too big of a deal to some, but as a genderfluid person who goes by they/them pronouns, it really means a lot to me. It's amazing for me to be able to make a sim of myself, and be able to have them use my pronouns!
(Idk if I'm making no sense or smth, or if there's a better place for me to say this, but uhh,, yeah!)

No, I currently don't know of a way to allow individual sims to be identified as non-binary, so I am limited to the neutral for everyone approach. As I stated above, I am not very optimistic about the possibility to do this, so I think this is the best that can be offered.

I am glad that you appreciate it. This isn't a mod I personally wanted nor need for the type of sims I make, but I did feel an obligation to make it as it wasn't a big deal for me (actually, a fun challenge; english grammar is not trivial) but very important for some people. And being gay I do know the feeling of not being taken into account, and it's much worse for you.

Remember that it will need to be updated with most new content releases, I'll try my best to update it as soon as possible.