[i18n] Replace php arrays for json/yaml files for language `key => message` definitions

[lcharette]

Title is pretty self explanatory. Yaml/Json file are easier to manipulate and validate than the arrays, which needs to actually be be included and could pose a security risk.

[Silic0nS0ldier]

Re the JSON, would be nice if a JSON Schema definition could be created to validate it. Also helps in supports IDEs like VSCode as you get autocompletion.