Here's a solution that is known to work. It's a little technical though. Don't get scared off from it! You can do it.
- Go to "/User/<user>/Library/Application Support/Origin/Installer Data/" (where "<user>" is your user name on your Macintosh. The "Library" folder may be hidden by macOS. You can easily reveal it though. See: https://answers.ea.com/t5/Technical-Issues-Mac/135dec40-90f690cf-00000075-495f32a0-Updated-patch-now-can-t-open/m-p/8934130#M30190
- Once in "Installer Data" look for files that end in the extension "plist". The file names will be something like: 1011164.plit or 1070252.plist. The exact name will change depending on what edition of The Sims 4 you have along with which Packs you have installed. Open each of these PLIST files. You can use the TextEdit.app in your Applications folder to open these files
- Look for "<key>Locale</key>" followed by "<string>en_US</string>" (or some other locale)
- Change the "en_US" (or whatever is listed there) to your preferred language. Refer to the list below. The form and case must match exactly
- Save the changes to ALL the PLIST files you've changed
- Make sure the Origin client GUI is also set to the desired language for both the Origin client (Origin -> Preferences or Command+, ) and The Sims 4 (open the Game Properties, then the Advanced tab)
- Launch The Sims 4. You may also need to perform a "Repair" of The Sims 4 if you still get the error or if The Sims 4 launches but no text is visible
Let me know if this doesn't work.
This list is the list of supported languages for The Sims 4:
English (US): "en_US"
German: "de_DE"
French (France): "fr_FR"
Russian: "ru_RU"
Polish: "pl_PL"
Swedish: "sv_SE"
Spanish (Spain): "es_ES"
Italian: "it_IT"
Dutch: "nl_NL"
Norwegian: "nb_NO"
Danish: "da_DK"
Finnish: "fi_FI"
Portuguese (Brazil): "pt_BR"
Czech: "cs_CZ"
Traditional Chinese: "zh_TW"
Simplified Chinese: "zh_CN"
Japanese: "ja_JP"
Korean: "ko_KR"
You must choose from this list for this change to work. Anything outside of this list will continue to have the problem.
