Solution : https://service.sap.com/sap/support/notes/77236 (SAP Service marketplace login required)
Key words : 
transaction smlt 'schedule import -> special imports -> import development classes ->, select -> special imports -> import ison, additional key words language transport, language importuse transaction smlt, partially competing inspection mechanisms, initial situationin customer systems, recognize customer software developments, translation systemthe translation occurs, /general/r3server/language/note, standard language files delivered
Related Notes : 
       
| 101220 | Current note on language import 3.1I (+ SR1) | 
| 78015 | Current note on the language import 3.1H | 
| 63805 | FAQ: Country-specific Customizing settings | 
| 62024 | Current note for language import 3.0F | 
| 48324 | Double entries in DD tables after language supplementation | 
| 38930 | Current note on the language import for 3.0D |