Thank You very much!
config.asm
%define VARIABLE_KERNEL_LOCALE en_US.ASCII
kernel.asm
%include "config.asm"
%push
%defstr %$kernel_locale VARIABLE_KERNEL_LOCALE
%strcat %$include_kernel_locale, "locale/", %$kernel_locale, ".asm"
%$include %$include_kernel_locale
%pop