To find which files are erroring, add
console.error('json error in file', path.join(coreLanguagesPath, lang, namespace + '.json'));
Before each of the following lines of the linked file:
Line 108: https://github.com/NodeBB/NodeBB/blob/master/src/meta/languages.js#L108
Line 140: https://github.com/NodeBB/NodeBB/blob/master/src/meta/languages.js#L140