The language "en-UK" is hardcoded in HumanTask handlers

jBPM5 requires the use of String parameter language for most operations to interact with HumanTask features. However the value "en-UK" is hard-coded in the handlers so users can not define another language, unless them override our handlers implementation to define a different value (which is not a good solution).

We should be able to provide the setup for this attribute programatically in handlers or externally by process definition.