First translate and generate new language from https://translator.appsbd.com (You can also use POEdittor to generate new .mo file). After translation, just download the .mo file.
Step 1 : put the file into language/override folder ( If you put override folder then it won't update in any future update of this app, you can also put that file in default folder instead of override folder, but it might be change in future when it app adding that language officially).
Step 2 & 3 : Open the file language/languages.json and add a node like the example bellow:
N.B : Don't add if the language is already exist in the file