From setting menu load a required translation after that goto import/export under Translation menu from that select Export Translation :

1) select your language

2) select file format i.e. PO File

3) select custom module in Module to Export

and then click on export you will get your translation file.

edit that file with your string and put that file in your module directory inside the i18n folder.

after that upgrade your module and load a translation again.

you will get your translated terms.