You’ll need to add the path to the list of LocalizationPaths so that it can be found. Given you’ve put it in Engine, I imagine DefaultEngine.ini would be the one you’d want to add the path to:
[Internationalization]
+LocalizationPaths=../../../Engine/Plugins/MyPlugin/Content/Localization/{targetname}
I’ve assumed that’s your path; fix it if needed, and replace {targetname} with your actual localisation target name (the folder containing your manifest, archive, and locres files).