Hi Hosein,
Thanks so much for your report!
Polylang’s default flags have an intrinsic size of 16x11 px. Since they are not SVGs, they will be pixelated when scaled up.
You can use custom flags in different file formats if you want to:
https://polylang.pro/doc/can-i-use-my-own-flags-for-the-language-switcher/
Best regards,
timmse