Not through Oracle OLAP alone. This is more of a Reporting issue and can be done via db view or using reporting tool features:
a) Mapping is restricted to a single column. So Long and Short Descriptions need to be mapped to a specific column in source table/view. If your condition is static or related to backend dimension attribute value conditions then you can define the logic in a source view and map both desc attributes to the special column.
define column green_ldesc_else_sdesc in dimension view....
case when product_alias.color = 'GREEN' then product_alias.long_desc else product_alias.short_desc end as green_ldesc_else_sdesc
Use this column to map to short_description/long_description attributes in olap aw dimension mapping.
b) In OBIEE Answers, you can expose the relevant fields in the reporting layer or Presentation layer of rpd and use it in a case statement to implement your logic.
CASE WHEN "OLAP Presentation Layer Name"."Product"."Level Name" = 'CLASS' THEN "OLAP Presentation Layer Name"."Product"."Long Description" ELSE "OLAP Presentation Layer Name"."Product"."Short Description" END
Looking at your subject line, if what you're looking for is support for Multi-Language, then yes, multiple languages (descriptions) are supported for Oracle OLAP. You need to refer to the OLAP Users Guide for more details.