I couldn't tell from the description if you've already tried this, but the following Liquibase documentation specifies several locations where it will pick up additional jars. Those locations include:

• The current working directory.
• A liquibase_libs directory in the current working directory.
• A lib directory in the Liquibase install location.
• Inside any .zip or .jar files in the liquibase_libs or lib directories.

Try copying the orai18n.jar to one of those locations.