There's no way to use a custom localisation system with the info.plist strings.
That part of your app will have to use iOS's default localisation mechanism.
This is how to localise the location request description with iOS's built in strings file localisation system.
// English.strings file
"NSLocationAlwaysUsageDescription" = "English description";
// AnotherLanguage.strings
"NSLocationAlwaysUsageDescription" = "ajbdknfuied wibnrf";
EDIT: At everyone down voting me. The question asked about using a custom "custom localisation system". They explicitly said they did not want to use the built in localisation system, but instead their own custom one. That is why I said it was impossible.
Localising NSLocationAlwaysUsageDescription
is completely possible. Using your own custom localisation system to do it is not.