I'm guessing the seller didn't specify whether it was english or not. but usually the seller will say "in japanese" or "japanese" if it is in japanese. so my best guess is that it will be in english. But the japanese tamas aren't that hard to de-cipher, many of the useres on Tama-Talk use the iDL and such just fine without knowing a single character of japanese writing. so even if you do get shipped a Japanese gotchi you can just easily tell your mom which icon does what and what to do if it calls for you... etc. I myself wouldn't let my mom baby-sit my tamagotchis, she is a great mother, but tamas she can't keep track of or remember to feed-care for it. so I would suggest pausing it instead of having the chance of death. Mothers are busy people!
my mom recently let my iDL die when she found it at home, told me she'd care for it, and BAM, dead. so I feel that mothers aren't the best tama-carers.