Commons Lang uses Maven or Ant as a build system.
You may also be interested in the upgrade notes:
To build a jar file, change into the root directory of Lang and run "mvn package". The result will be in the "target" subdirectory.
To build the full website, run "mvn site". The result will be in "target/site". You must be online to successfully complete this target.
Further details can be found in the commons build instructions.