Hey Lenin,
I provided a detailed step-by-step on how to get those .po files working in the other thread you replied to, so definitely check that out when you have a moment!
In regards to the standards of the PO file, I’d recommend referencing the translation files that we include with the plugin’s default installation. These can be referenced in your WordPress folder structure under plugins > mailchimp > po.
Try selecting a file there like “mailchimp_i18n-en_US.po” and opening it up in your text editor or POEdit. I prefer POEdit as it provides an easy column-based interface to edit your translations, so it helps me to ensure that everything is lined up correctly and being recognized in the proper format.
If you have anymore questions, let me know!