What language? There are various community magazines - ici londres for the French community for example.
Gumtree, nannyjob and netmums are alternative routes or, if it's an EU language, them someone from within that country with a good command of english, assuming you want them to speak their native language with your DC.
There are a couple of agencies which actively seek bilingual nannies who I used to be registered with but also place monolinguals so aren't specialist bilingual agencies.
Look on greaupair.com, nannyjob.co.uk, or a variety of other sites for girls from Germany. Or... place an ad in Germany in English and see what you get. I would think this is a pretty easy thing to find to be honest. Lots of Germans would like to come live in the UK and brush up on their English.
My post was assuming that you are looking for live-out nanny, qualified or experienced with the additional bonus of speaking German. Qualified bilingual nannies are not very easy to find and usually cost a bit more.
Reading the other posts seems that other posters talking about apairs or young girls looking for live-in position, in which case i agree that it is easy to find one.
There are many German mums here maybe worth to give them a shout in 'chat' section?
German shouldn't be too bad to find. Germany has a fairly healthy nanny market itself so you could advertise there. Agree that qualified and/or experienced bilingual nannies are harder to come by but German is one of the easier languages to find! And advertising in Germany is definitely an option but research the standard qualifications first.
Felicity nannies, pearl childcare and royal nannies all place qualified and/or experienced nannies with additional languages iirc and eden and imperial might also be worth a shot as they have good European desks. Felicity nannies is an agency actually based in Germany who place nannies all over and Elizabeth and Mirja have always been very helpful to me as a nanny.
I wouldn't go to great au-pair personally as you are more likely to find younger and less experienced candidates. Gumtree and nannyjob I have used as a bilingual nanny. Netmums will give you candidates from your local area. Another alternative is to advertise at the German school in case anyone has a nanny they're letting go.
It will almost certainly cost you more but is definitely worth it.
If you're willing to settle for less than a ft or pt sole charge qualified and experienced nanny and just want some support with the language then there are other options but the places mentioned above are probably a good start.
You can find nannies on greataupair.com too. You may have to weed through a considerable number of au pairs to get there. But, I know a few nannies who have profiles there. And, greataupair has a great search facility, making the whole weeding process so much more bearable.