I am looking for solutions to how to handle the i18n localization efficiently in automation testing.
Firstly, welcome to the forums.
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With your current questions, we don’t have enough context to know what you already know or don’t know, so it is impossible to guide you without just telling you the answer (which we won’t do).
It is pretty typical on here for people to share a codepen / repl.it / jsfiddle example of what they have tried so that anyone helping has more of an idea of what help is actually helpful.
Please provide some example of what you’ve tried and I’m sure you’ll get more help.
Happy coding 
It depends on what aspect of the localization you’re trying to “handle”.
- Your test environment should have a consistent locale, so the functional tests that involve localized strings, numbers, etc can just test against the expected result in that locale.
- You can write tests for your localization service itself, testing with specific locales and expected results.
- You can write tests and/or a linter which tests that all new elements properly call the localization service.
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We are storing it in separate properties files as key value for every languages and utilizing the files for specified language
That’s pretty typical.