Is it possible to specify the Cyrillic alphabet in C language conditions?

Question

C++ / C#

if (tolower(symbol) == 'a' || tolower(symbol) == 'а') {
    addition();
  }

above is the condition that the compiler complains about
|| tolower(symbol) == 'a' - Cyrillic here
tolower(symbol) == 'a' - here Latin
removes the condition with Cyrillic, compiles and does not swear
, what could be the problem? maybe I wrote the condition incorrectly?
main.c:21:51: warning: multi-character character constant [-Wmultichar]
if (tolower(symbol) == 'a' || tolower(symbol) == 'a') {
The compiler error itself