Unfortunately I can't tell you how popular the Paris base is, but yes, you have to speak French if you would like to apply there. (Although there are people at foreign bases who don't speak the language and still managed to get there as a newbie...there has to be a way around it, but don't know how...)
If you don't speak french, apply to any other base where you speak the language and you can transfer after 6 month.
"Secondment" means that you are on an English contarct, but not actually working in England. Theoretically you are based at one of the English bases and you are only "seconded" to your actual base.
Madrid and the Swiss bases have their own contract.
Hope this all made sense...