The Redis server itself stores all data as a binary objects, so it is not dependent on the encoding. The server will just store what is sent by the client (including UTF-8 chars).
Here are a few experiments:
$ echo téléphone | hexdump -C
00000000 74 c3 a9 6c c3 a9 70 68 6f 6e 65 0a |t..l..phone.|
c3a9 is the representation of the 'é' char.
$ redis-cli
> set t téléphone
OK
> get t
"t\xc3\xa9l\xc3\xa9phone"
Actually the data is correctly stored in the Redis server. However, when it is launched in a terminal, the Redis client interprets the output and applies the sdscatrepr function to transform non printable chars (whose definition is locale dependent, and may be broken for multibyte chars anyway).
A simple workaround is to launch redis-cli with the 'raw' option:
$ redis-cli --raw
> get t
téléphone
Your own application will probably use one of the client libraries rather than redis-cli, so it should not be a problem in practice.