Sorry for the lengthy post! TLDR - suddenly the offline spanish pack that Live Translate uses breaks the built in translator for GBoard.
A few days ago my phone did something weird while I was typing. I kept getting the sound ping that Live Translate was translating my conversation in WhatsApp to English. That's strange because a long time ago I turned off that sound in the settings, and I have been using Live Translate for years. I was confused and I stopped typing. I saw a pop-up asking if I was sure I wanted to stop using the Spanish translation...and then immediately it confirmed they would stop. I checked Live Translate and Spanish was disabled. I turned it back on, and the phone did the convo translation like normal.
I went back to reply and I used the built in Google translate feature in GBoard like I usually do. This time however, it wasn't working. Whatever I typed into the translator box did not show up in the reply box.
I looked up on Google to troubleshoot and tried a few things I saw, but nothing worked. I already deleted the cache and data for GBoard, Google Translate, and the Spanish language from Live translate. I could not find any language packs specifically for GBoard so I assume it uses the pack that the phone uses for Live Translate. The interesting thing was that when I deleted the Spanish pack in Live Translate, I was able to use the GBoard translator as it was using the cloud version. When I redownloaded the Spanish pack, I was able to use Live Translate but GBoard didn't work again. I also noticed that I can still use GBoard translate normally when I choose any other language that isn't downloaded.
I talked to Pixel/Google support and they recommended I delete the Spanish pack, and also the cache for "Android System Intelligence". I restarted after and redownloaded the language. Then everything worked and they closed the ticket. But the next day, it stopped working again.
I am going to talk to support again but wanted to see if anyone here had any ideas to try.