Object literal and JSON are the same thing (correction: see Quentin's comment)
IF you're searching for a value then JSON can have better performance because it's a hashmap implementation, so the lookup time would be O(1).
JSON is also more flexible, since you can have arrays in your JSON, you can have named keys, etc.
That said, I wouldn't worry about the performance gains here. They're probably negligible.
If you're trying to map your HTML pages to a language, you can either store the URL in a JSON keyed by language abbreviation ('en', 'de', etc.) or you could use a convention for naming your files (index-en.html, index-de.html). It's your choice.