Solution : https://service.sap.com/sap/support/notes/1159021 (SAP Service marketplace login required)
Key words : 
subsequently import language parts, language imports language maintenance, required postprocessing steps, copy customizing texts, general consistency check, transport tools - check, running nls configuration, red traffic light, code page reason, online correction support
Related Notes : 
       
| 1375438 | Globalization Collection Note | 
| 1366014 | RSTLAN_LANGUAGE_CHECK: Incorrect supplementation mode | 
| 1309530 | Required adjustment for the report RSTLAN_LANGUAGE_CHECK | 
| 1156507 | Language supplementation, RSREFILL and client maintenance | 
| 195442 | Language import and Support Packages | 
| 110910 | Deleting the language load | 
| 42305 | RSCPINST (I18N configuration tool) |