To translate the theme you have to:
.po file with a title according to your language abbreviations and associated country. For example if you want to create an American English translation, the file will be named
.po file from Poedit, the application automatically generates an additional
.mo file and you have to saves it in the same directory on your language folder.
The translation code must match the language code of wordpress. More information than here. For example the Spanish language code is es_ES but if you have installed wordpress in Argentine language the code will be es_AR.
In your language folder ( ../wp-content/themes/ong/languages ) you will have for example: