Magento 2 – What Does i18n Mean?

Does this mean there is no such thing as a module scope anymore?

Yes

If so, what happens if two different modules define different translations for one string?

Magento 2 load translation from all modules and then load from current (based on current controller). In most cases this resolve problem.

I'm thinking of ambiguous words like "state" that will be translated different depending on the context. How does Magento 2 deal with these?

Looks like is no big issue there. In any case Magento can add inline context like State (Address), State (order)

There is still some prototype.js left over from Magento 1, now mixed with jQuery (in noConflict mode)

That means:

• $ is the prototype selector that returns one element by its id
• $$ is the prototype selector that returns an array of elements by CSS selector
• jQuery is what would be $ in a pure jQuery environment and returns a jQuery wrapper for an element by CSS selector