The stock config.tcl already does what you are asking for. Look to the commented link for further details.
ns_section ns/parameters
# Unicode by default:
# see http://dqd.com/~mayoff/encoding-doc.html
ns_param HackContentType 1
ns_param DefaultCharset utf-8
ns_param HttpOpenCharset utf-8
ns_param OutputCharset utf-8
ns_param URLCharset utf-8