Guesing you are in a Unix system ...

1- Do i set the variables with the motor started and from informix?
You´ll have to bring Informix down , make the changes
and bring it up

2- which are the possibles reasons that provoked the changes of these variables.
I really dunno ....

3- How can i do to see this variables, i mean what command can i use to see the content or value of these variables, before setting it with the values you sent me.

If Informix user brings the database up
edit Informix .profile
and set the variables there
like
CLIENT_LOCALE=EN_US.CP1252
DB_LOCALE=EN_US.CP1252

export CLIENT_LOCALE DB_LOCALE

you can see in your shell with

env|grep LOCALE