The short answer is focus on what is best for your users and what they expect, but there are a couple of things you need to do for best indexing in some cases.
To answer your last question first, if your server can respond to a URL and browsers fetch it successfully, Googlebot and indexing will be fine. Want to use UTF-8 for fully localized URLs? Go for it. Want to stick to ASCII? Sure. What's best for your users is the answer.
Trends across languages is a very odd request. Take the string of characters "chat". In English it's used in many different ways, a verb, a noun, etc. Same string in French means cat. You really wouldn't want to look at the trends of this string by combining English and French.
Now your biggest question: a search engine tries to identify the language of the pages in its index, and also the language the searcher is using. To illustrate:
1. The "chat" example is perfect for this, but also any number of queries.
2. Take someone searching from Switzerland. Wouldn't it be better to show them pages in their preferred language, be it German, French, or Italian?
3. Take someone looking for a specific business. If that business has pages for its UK, Australia, and USA subsidiaries, it would be great to show searchers the right country page if they're searching from the UK, Australia, or USA, even if all are using English queries and these localized pages are also all in English. For that, you'll need hreflang annotations, which also works across languages in the same country (Switzerland) or across languages (global pages in English and Spanish).