login_script.pot file is to create more languages which are not yet available in system.
While PHP login script already available in various languages so in case you want to activate any language you have to go General settings and activate your required languages.
To edit those languages you need to use poEdit software, just edit that certain language .po file and make changes save and upload it should work fine.
Which language are you trying to use? Also for English language you don't need to use these locales/ you can just edit your required file and change string to your own string for english language.
Thanks.