CLI - How to download and combine mutiple files of same language into one file?

current download config:
{
“download”: {
“includeSourceLang”: true,
“metadataFileJson”: “metadata.json”,
“files”: “public/locales/${lang}/${file}”
},

i want to acheive something like this: (combined same languages into single file)
public/locales/
ar.json
bg.json
cs.json
de.json