What do you mean when you say "Google's keyboard"? I use Gboard and frequently swipe sentences which mix Spanish and English just fine. You enable "Multilingual typing" on the specific keyboard and then tick off languages from other installed keyboards to enable them. Maybe you haven't tried gboard for a long time?
There's some internal matrix of "Supported" clean movement, mostly between languages with an extensive shared vocabulary. You can enable Multilingual Typing theoretically anywhere, but it only really "works" among languages that share a layout and character dictionary.
But if you need to loanword a non-common word from English into another language (e.g. Hebrew), Gboard just can't do it. If you change character sets, you change entire dictionaries. SwiftKey has a unified dictionary among all languages and makes a best guess approximation based on the current language selected and then tries the other languages you have configured.
This means that if you're going along in, say, French, but then need to reference a word in Hebrew, you can swipe over to Hebrew, bang out a word, then swap back to French, type words in English (which get autocorrected in english) and then continue in French. This isn't really feasible in Gboard without a lot of back and forth.
