What do the three checkboxes on the Interface translation import mean?
Treat imported strings as custom translations


Will add to the Site the translations strings present in the .po file as custom translations.


Overwrite non-customized translations


All the strings that comes in the .po file will overwrite the existing translations that are not customized in the Site.


Overwrite existing customized translations


The customized translations present in the Site will be overwrite with the translations present in the .po file.