Hello everyone! I hope someone can help me on this subject. I've been trying for the past few months to modify the existing Mexican Spanish localization with Argentinian Spanish slang and cultural differences just for personal use. I've been using just S3PE because i'm virtually illiterate in this aspect and it's been the program I'm used to, however:
After going through the entirety of DeltaBuild0 and DeltaBuild_p02 (World Adventures) I've noticed that many strings are missing in the MX localization, i.e:
These are random strings for Ambitions text in Spanish, as you can see there are lots of empty spaces
These are the same strings in the original English, showing a lot more text

I decided to check the other expansion packs (GameData -> Shared -> Packages -> FullBuild) and while some of them have completely different strings in spanish vs english, some packages straight up do not have a Mexican Spanish localization. This leads me to wonder where my game is taking the translations from, because in game I have no problem seeing text, but I'd like to know:

Should I "fill in" the empty spaces with my own translations from scratch or are these missing strings stored someplace else? And what do I do with the missing MX spanish STBLs?

I'd be very grateful for any help since I am very useless at coding and modding!

I'd recommend using the program S3SE (Sims 3 String Editor, same authors as S3PE) to manage strings, it's very useful for comparing string tables.

My theory is that the localisation is coded to use the ES-ES Spanish whenever an ES-MX is left blank - you could confirm this by comparing a missing string that you're able to locate in-game with the ES-ES string to see if they match.

It could also be that there are unused English strings that weren't translated to other languages. I know there are at least a few unused strings, but I'm not sure it's as many as you're seeing here.
Edit: okay actually looking at the missing strings in the screenshots, it's probably not unused game strings with them being object descriptions :p

Thank you so much! I have downloaded other string editors before but they were too complex for my pea sized brain but I'm gonna give it a try and see if I can figure it out °_°
Actually this might be the case, throughout my gameplay I have always found some Castilian Spanish strings but I just chalked it up to being a choppy translation, in this case (where there is no MX localization) how does the game manage to display two languages at once? Can I copy the ES string and change the first two numbers to make it MX spanish or will that bork up my whole game?

You can certainly give it a shot - as long as it's in a new package (not editing the original directly) there is no harm in testing it.
I know some players do this with mods that have low quality translations, to use the English set regardless of the game language.

If it works as I suspected, the code that handles localization is testing for empty strings and redirecting them when found.
Meaning that if you fill in the blank ES-MX strings, they would take priority and no longer redirect to ES-ES.

That's very interesting- Thank you so much for your help! I have downloaded s3se and copied all strings so I'll proceed with the translation and cross my fingers!!