Answer to your question:
Q1: For a Multilanguage website, should I have the RelativeUrl parameter set to "en", "es", "de" for each of the languages. ?
Add-TtmMapping -Id MAPPINGID -CmEnvironmentId CMENVIRONMENTID PublicationID "tcm:0-8-1" -WebApplicationId WEBAPPID -RelativeUrl en
Add-TtmMapping -Id MAPPINGID -CmEnvironmentId ...
The issue here was that the content was being cached at multiple locations and so the services and the webapp needed a proper restart and in the correct order. We are using JMS for the CD services so here is the order in which we restarted and it worked
JMS Caching services
DXA Model service
You can refer to the following document links for the configuration.
Adding Experience Optimization to your .NET API (RESTful) Role
Configuring a .NET website for Experience Optimization
Configuring Experience Optimization behavior with the smarttarget_conf file