Thanks, I got the name of field values with code like this:
+++Define Field: FIELDS/+Name
Set TestingL<Invoice Status> = <Invoice Status.Complained>
Set WorkL<JuhoheTest.Field Name> = <JuhoheTest.Field Name.Invoice Status>
+For Each Field TestingL
+++Set Value To Current Field: FIELDS/+Name
++Name Defined Field: FIELDS/+Name, WorkL<HelpString1>, .Language
If WorkL<HelpString1> == WorkL<JuhoheTest.Field Name>
+For Each Field Value
+++Set Value To Current Field: FIELDS/+Name
++Name Defined Field: FIELDS/+Name, WorkL<HelpString1>
The human readable name of the value is stored in field WorkL<HelpString1>
The discussion linked by Teemu Simolin contains the solution for my problem.