set like following.
==web.xml==========
<'web-app>
<'context-param>
<'param-name>weblogic.httpd.inputCharset./*<'/param-name>
<'param-value>Shift_JIS<'/param-value>
<'/context-param>
<'context-param>
<'param-name>weblogic.httpd.inputCharset./rus/jo*<'/param-name>
<'param-value>ISO-8859-1<'/param-value>
<'/context-param>
<'/web-app>
===================
but its not good way.
its for old version of WebLogic.
version 6 or sth.

the certain way is
prepareing a SetCharacterEncodingFilter
and set about that in web.xml.

the best way is
doing sth in META-INF/weblogic.xml.
but it didnt work well.
==weblogic.xml==============
<'?xml version="1.0" encoding="UTF-8"?>

<'weblogic-web-app xmlns="http://www.bea.com/ns/weblogic/90">
<'charset-params>
<'input-charset>
<'resource-path>/*<'/resource-path>
<'java-charset-name>Windows-31J<'/java-charset-name>
<'/input-charset>
<'/charset-params>

<'!-- doesnt work well.
<'input-charset>
<'resource-path>/*<'/resource-path>
<'java-charset-name>Windows-31J<'/java-charset-name>
<'/input-charset>
-->

<'/weblogic-web-app>
============================

ref:
http://www.beasys.co.jp/e-docs/wls60e/ja/relnotes_j.html#1071437