When using Google Cloud Load Balancer, you can add certain fields to custom headers such as {client_city}, which (surely?) are unicode strings.
I cannot find in the documentation how these are encoded in the header (bearing in mind that HTTP headers are ASCII only, or possibly Latin-1 according the "obsolete" productions in the HTTP RFC).
Since I'm in a city with an ASCII name, I've not been able to run a test from my local box.
Google has a document for this:
client_city:
Name of the city from which the request originated, for example, “Mountain View” for Mountain View, California. There is no canonical list of valid values for this variable. The city names may contain US-ASCII letters, numbers, spaces, and the following characters:
!#$%&'*+-.^_`|~.
Some headers can be Unicode, whereas some well known headers are Ascii.
https://cloud.google.com/load-balancing/docs/backend-service
