The \Magento\Framework\App\Response\Http::representJson() does not specifies a JSON response's charset and removes a previously specified charset, so not-latin characters are rendered incorrectly by all the modern browsers

[dmitrii_fediuk]

Google Chrome

Firefox

https://github.com/magento/magento2/blob/8fd3e8/lib/internal/Magento/Framework/App/Response/Http.php#L140-L151

[dmitrii_fediuk]

See also:

• How to return a JSON response from a controller
• How is the «Content-Type» HTTP Header added to a JSON response?