An entry in Fortnightly Topic Challenge #45: Flags
The answer is a sentence.
Hint 0 before the bounty expires (the length of the words in the answer).
As I said in the comments @Alaiko's answer is definitely on the right track, the problem is the method they use to compute the final answer. Using their method the answer would have a lot of letters while the true answer has only ten letters (see the previous hint)! Try to find a method that produce a ten-letter answer.
There is something "strange" in @Alaiko's answer, it is about a mashup flag in the third row of the grid. After you fix that, it would be easy to find the way to compose a ten letters sentence out of the grid in their last spoiler tag.
In their answer @Alaiko states that:
- For the first row, the trend seems to be that the first letters of one country also appear as the last letters of the other country.
- For the second row, the names of the two countries share the same first few letters.
- For the third row, the shared letters of the two countries either appear at the start of both countries or at the end of both countries.
Isn't the rule for the third row a bit strange? The sequence would sound more natural if the last rule was "For the third row, the names of the two countries share the same last few letters". Also, the only flag in the third row that follows the "shared prefix" rule is Latvia+Laos, that sounds more like an exception than a rule...