While not as complete of an answer as I'd like, I was able to improve my typing experience vastly by doing the following:
First, it is important to understand the difference between auto-correct (or auto-replace, as it is known in the settings) and predictive text. Predictive text, when enabled, is what is shown in the black bar above the keyboard. Auto-replace automatically replaces whatever string you've typed with the most probable string pulled from a dictionary file.
By going into my keyboard settings and enabling predictive text AND auto-replace for ALL installed input languages I was able to greatly improve the function of the keyboard. For whatever reason, it would seem that unless auto-replace is enabled for all languages, the probability algorithm doesn't effectively limit it's results to the current input language.
I tested English, German, Turkish and Spanish and was able to receive useful predictions and auto-replacements for each language.
That said, it isn't a perfect solution. Text predictions at the beginning of a sentence are more likely to contain unhelpful suggestions (probably due to limited data fed into the prediction algorithm), and intruders do still occasionally make their way to the front of the auto-correct list.
For this reason, it's probably important to absolutely avoid lazily entering text in a language other than the currently selected keyboard in order not to muddle up the predictions.
It is still unclear to me how the dictionaries are actually stored on the device, so hopefully a more in-the-know user can provide a more satisfactory answer in the future. From the observations I've made, it would seem that a single, combined - language dictionary file is being used.