Solution : https://service.sap.com/sap/support/notes/61522 (SAP Service marketplace login required)
Key words :
create job sap-language-import-job-1-, directory~/general/r3server/language/note, select import mode 'full import', 0d including language upgrade, additional key words upgrade, select 'special imports' -> 'import, transaction smlt -> language status, full language import, select 'schedule import', start transaction smlt
Related Notes :
| 38930 | Current note on the language import for 3.0D |