Even though sub-string search is a slightly offtopic for this post I'll go on with this.
As a native Finn it's quite hard to use Gmail search for anything in our native language.
Just an example. If I want to search for, as an example, invoices (which many companies send directly to your email) I simply cannot do it.
The reason is this:
"Tässä laskunne" - "Here is your invoice"
"Saitte laskun" - "You've received an invoice"
"Puhelinlasku" - "Mobile bill" (the monthly invoice)
The word for invoice in Finnish is "lasku". And there are immense amount of ways to morph the word. And that is with every word. So in order to search for those kinds of mails we usually just search in the basic form. In GMail it's impossible so it's a pure guessing game.
I think this shows nicely that when developing something and thinking "This feature is not really needed" you might accidentally ruin the whole main feature for use cases you're not familiar with.