Update translation

Hello, I made a custom translation on local last year for erpnext and frappe version 8, now when I update the erpnext to latest update I lost the translation I made.
The good thing I still have the translation on local, but the problem when I migrate the old translation to the new version, there is a new features/field not translated.

my question is; how can translate only the untranslated words, form an old file to the new one.

thank you.