In some languages, one vowel decides how an entire word must sound

Turkish, Finnish and Hungarian all use vowel harmony, a rule where every vowel added to a word, including suffixes, must match the 'front' or 'back' quality of the word's first vowel. That's why the Turkish plural suffix is spelled '-ler' after front vowels but '-lar' after back vowels, the ending itself has to shift to fit the word.