My son is 25 months and can't understand some basic commands/phrases/words. Is he developmentally challenge?

He's a bilingual child. He's got both languages spoken to him. He stays with my mom and dad in the day time and they only speak to him in spanish (does not speak english). at the same time i've got other people, aunts/uncles/cousins and myself talking to him in english. could this be the reason? I've been told that would be the case, but just recently someone pointed out that there could be something wrong with him. Of course this persons is NOT an expert and should stop comparing my son to her grandkids. Sorry.. a got a little personal there, but had to vent. Any insight to this is welcomed. Thanks!