To change the log language for Liberty use the following approach:
Create the file jvm.options file and add the following line: -Duser.language=en (for english).
Restart the Liberty Server.
Comment:
There are different places where you can define the jvm.options file, see
https://www.ibm.com/support/knowledgecenter/SSEQTP_liberty/com.ibm.websphere.wlp.doc/ae/twlp_admin_customvars.html