Help with my forum and some characters
 

I have this problem on my forum:

My site is in Spanish, when somebody types an accent (a', e') or the spanish letter ? you get an interrogation sign (?) instead. This happened with the last version an it is happening with 3.8.x as well.
Can you please tell me how can I fix this?.

I do haver the last version but it has been happening since version 3.6.x

Appreciate any help at all.