Grandma and toddler can't understand each other (speak 2 different languages)?

My mom, sister and I came to the US from Spain when we were both 5, my mom never learned english, but for us it became our first language pretty fast, knowing how to express ourselves better in english than in spanish. When we turned 18 my mom went back to Spain, my sister, who now 25, has a 4 year old that doesn't speak any spanish at all and when we try to tell him "say this..." he just says "I can't". On July she's going to take him to Spain to see our family for the first time, problem is none of them speak any English and the tot doesn't speak any Spanish, would this be an issue?