Just so you know, a Sinhala translation is already in progress.
Maybe it would be a good idea to join that team.
First, you need to create .po files for the Sinhala translation of WordPress. You're going to need four of these files. They will be called
If all you want to do is translate the back end, you only need to provide
admin-si_LK.po. If you already have some or all these files, great.
Second, you need to generate the corresponding .mo files by compiling the .po files. You can do this by using Glotpress. https://make.wordpress.org/polyglots/handbook/tools/glotpress-translate-wordpress-org/
Third, you need to put them into the following directory in your test WordPress installation. Then you can test and edit.
Finally, you can contribute your translation back to the WordPress core via Glotpress.
As of WordPress 4.0, you wouldn't use a plugin to provide a translation. Instead, you'd contribute your translation directly to the WordPress code base,