Solution : https://service.sap.com/sap/support/notes/933836 (SAP Service marketplace login required)
Key words :
löschen nicht erlaubtalso add, func hrhap_c_it1000_langu_deletefunc hrhap_c_vc_copyfunc hrhap_c_vc_modify, message class hrhap00_template message 024, delete language versions, package interface paoc_if_hap_template, message text, package paoc_hap, support package, infotype 1000 objects, 3 function modules