If you need to install UTF8, then install the related package

Install bos.loc.utf.* packages for your locale. E.g.:

#lslpp -L bos.loc.utf.*
  Fileset                      Level  State  Type  Description (Uninstaller)
  ----------------------------------------------------------------------------
  bos.loc.utf.EN_US          7.3.2.0    C     F    Base System Locale UTF Code
                                                   Set - U. S. English 

Then you will get UTF-8 locale in the output of locale -a:

#locale -a
C
POSIX
EN_US.UTF-8
EN_US
en_US.8859-15
en_US.IBM-858
en_US.ISO8859-1
en_US.UTF-8
en_US

For info tmux requires UTF8, else you got the following message

tmux: need UTF-8 locale (LC_CTYPE) but have ISO8859-1