I can only speak to Anglo-Saxon usage of french maids, but there are several reasons they were preferred, but before I got into these I would just like to say that the majority of french maids severing the gentry were not poor peasant girls, for the most part they were from the lower-middle or middle classes or sometimes the upper middle class (though those served mainly as governesses) and they were educated in schooling and manners, now of course cheaper maids from the lower classes were available, but they were mainly used by middle class English who were imitating what the upper classes were doing.
The first reason for their selection was their knowledge of proper french, in addition to the head ladies maid, they often helped as governesses (depending on their social station in France) or tutors to educate the younger Anglo-Saxon gentry. Secondly, they were thought to have a better idea of fashion, hair-styles, and general trends in france, which was very valuable to the lady of house, as most fashion was coming from France at that time. Finally, the French were seen as racially acceptable to the English, and though maids from India, China, etc, have been attested to, the vast majority of English housekeepers came from W. European countries, ie France, Belgium, Netherlands, etc... There were Irish housekeepers too, but they usually occupied to lowest tier of the staff.
To slightly expand on this, the servants of a household had a clear line of rank, the french maid, who primarily served the lady of the house, was in the upper ranks usually ranked about 3rd or 4th behind the butler, stable keeper/gardener and sometimes the cook. Her primary responsibilities were dressing/bathing/perfuming the lady of the house and accompanying here to various lady functions. The lady of the house, for the most part was able to choose the maid on her own, this bypassed the traditional hiring practice of having the butler handle that. So it seems a bit obvious if she got her pick, she would choose someone with her interests, fashion, hair styles, art, couture, continental manners, etc... which a french maid could provide.
Also at this time period there was much upheaval in France, leaving once upper-middle class families in destitute, for some taking a maid/governess job was a way to keep them out of poverty and to create connections with English society that could be useful.