https://www.w3.org/International/questions/qa-link-lang#flags
Flags represent countries, not languages.
Numerous countries use the same language as another country, and numerous countries have more than one official language. Flags don't map well onto these permutations.
In addition, flags have nationalistic connotations that may be unwelcome for people of other countries, even though they speak the same language.
While flags can be very appropriate for sites that are distinguished on the basis of region (eg. the amazon.co.uk site vs. the amazon.ca site), you should normally avoid them when dealing with links to pages that are just translations.
https://www.w3.org/International/questions/qa-link-lang#flags