Question: are prime numbers prime in all bases?
Answer: A prime number is a prime by defination, irrespective of base. We should distiguish between numbers, on the one hand, and numerals , on the other, which are used to represent numbers. So, e.g., 13 = 15(octal) = D hexadecimal = XIII = treize, in French word(s) = τρισκαίδεκα. It is prime, no matter how you represent it.