Swapped SIM card and phone gone ballistic

[sophie]

I removed my husbands SIM card from his Samsung and placed in my Huawei as he didn't like his Samsung. In my naivety I thought it was just a matter of switching on again and bobs your uncle. My Hauwei phone will have become his phone with all that was on his SIM card etc. NOT SO!!! Suddenly my older Hauwei is all in what we have now discovered is Croation. I think I've moved his contacts over but that's all. Do you think I should go on to Google/Youtube and put the Huawei back to Factory Settings and to quote "alphamike" re my other query start from scratch?