I considered this at one time, but because my mum lives on her own, I chose not to take this route. Instead, I took on a private person, who I knew, which is the safe option. I do use an agency, however, to give my live in carer a week off every month and it's not always the ones that speak good or perfect English that have been perfect for my mum. Just recently had one lady from Romania who spoke very broken English, and my mum doesn't speak English very well either because she was originally from Germany and her condition often makes her forget English, and this lady was brilliant with my mum. I would walk in sometimes and my mum would be chatting in a mixture of broken English and German, and the other lady was laughing and somehow saying something back which worked because somehow my mum could understand through the body language. I've also had a 19 year old in, who managed very well, and my mum liked her because she was very pretty.
I've had 2 English speaking carers, one spent most of her time crying when my mum had a go at her, and one who was older and very experienced actually threatened to leave after day 2.
My daughter was an Au Pair when she was younger, both times for small children, but that was her choice. So the drawbacks. How would you interview someone? You would probably have to pay their fare over, or at least to be on the safe side, pay them back when they arrived. If it didn't work out, you would need to pay their fare back again, regardless of how long they were with you. You may find that the laws are quite strict, for one thing, there is a good chance that someone from abroad will want more than £100 a week, plus you will need to pay towards travel home abroad when they get homesick if they are younger. There are laws which means even though you don't have to pay them a living wage, you are only allowed to employ them to work for about 30 hours a week, and if they are young they will have to have regular breaks.
Consider, why does it have to be someone from abroad? You are living there too, so if it didn't work out with someone closer to home, it would be a lot easier to give them a train fare home. My daughter's first Au Pair job was when she was 16 years old and the job was in Luton.
I think your best option might be to decide what you actually need help with. By the way, when my mum was at the start/middle stage of Alzheimer's. I enrolled her into daycare. She still goes now, but that's to give her carer a break. At the beginning, though she loved it, and looked forward to going every day, even days when it wasn't open which was the sad part, because she still lived on her own at the time and was upset when they didn't turn up because she thought they had forgotten her, even though I rang her every morning to tell her if it was a daycare day or not. They feed her there, bring her there and back again, there is an option for assisted showers and haircuts, they organize a pediatrician to go there when needed. She does like going still, but she tends to try and walk out again quite often, but my hope is that if there comes a time she needs to go into a home, she will still have a few social skills to cope in one.
If you do decide to follow through this route, my daughter was an au pair on Aupair.com and had no problems with that site, and the ladies that employed her got her through that site, so it's a safe one to get someone through.