There is simply no 1:1 correspondence, or only in a few special cases like The Faroe Islands, and maybe Nauru or Kiribati.
Homogeneous nation-states are an illusion, mostly carried by the similarity between big national languages and country names (Spanish : Spain, French : France, etc.). Furthermore, language borders rarely coincide with political borders to an extent that would justify equaling nations and languages.
Problem type 1: one language – many countries
- German is spoken as official language in Austria, Belgium (parts), Germany, Liechtenstein, Luxemburg, Switzerland (parts).
- Don’t start counting the dozens of options for Arabic (26 countries), English (59), French (29), Portuguese (10) or Spanish (21).
Which country flag should we take?
Problem type 2: minority, diaspora and suppressed languages
- Romani, Jiddish, Kurdish, Basque and other languages are spoken across several countries but do not have official status in any.
Show me one Kurdish speaker who’d accept being represented by e.g. the Turkish flag.
Problem type 3: languages considered dialects
- Catalan, Asturian, etc. are usually considered languages of their own, and at least for Catalan i know that there is a strong identification with being Catalan, not Spanish.
Yes, Catalunya has a flag. But then what about Andorran Catalan? And no, suggesting the Spanish flag would set a cat among the pigeons...
Problem type 4: one language – or two – or three?
- Hindi/Urdu on the one hand and Bosnian/Croatian/Serbian on the other are usually considered to be politically motivated distinctions with a rather weak basis in linguistic genealogoy.
Which flag to take for the undivided linguistic entity?
Problem type 5: one county – many languages
- Papua New Guinea has about 850 indigenous languages, 2 official languages.
- Nigeria has over 500 indigenous languages, 1 official language
- Indonesia has over 700 indigenous languages, 1 official language
- Brazil has around 230 indigenous languages, 1 official language
- South Africa has more than 35 indigenous languages, 11 official languages.
- Even a tiny country like the Vatican has more than one language.
So, which language gets represented by the Papua New Guinean, the Nigerian, the Indonesian, the Brazilian, the South African flag? All? None? Which flag should the other languages get, then?
And to open yet another can of worms:
Problem type 6: The residual category
Which flags should these languages get:
- Esperanto, Toki Pona, Elbish, Klingon, Na’vi, Solresol
- Russonor, Space Pidgin, Pitcairnese