Switch languages.json to JSON5

[XAMPPRocky]

Currently we're using a JSON parser to parse the contents of languages.json however this format is pretty unfriendly to contributors, with onerous requirements like no trailing commas (#982).

I'd like to move the parsing to using JSON5 as that is compatible with the current format, so it doesn't require a big change, while also providing a lot QoL improvements like trailing commas and comments.