Lost in translation

The French sentence ‘je n’sais quo’, which of course translates to ‘I don’t know what,’ is typically used to indicate that something has a certain indefinable quality, so indefinable that it’s impossible to, well, define it..
Let’s be serious for a minute – it means that it’s so indefinable that it’s impossible to do justice to it.
It’s sometimes used in conversation in Britain but not that often. If you are visiting a British flea market or ‘car boot sale’ (garage sale in the US), and if the person who is selling an item is boasting about it – is claiming that it has a certain indefinable quality, don’t expect him to claim that it has a certain je n’sais quo.
It just doesn’t happen, partly for linguistic and cultural reasons and partly because the kind of stuff that is sold at car boot sales hardly ever has a certain indefinable quality, a certain je n’sais quo.
Car boot customer, who is inspecting a used car battery charger or a baby pushchair (stroller in the US) which still has the original stains from the last time baby threw up, to seller: “Excuse me, does this have a certain je n’sais quos?”
Actually, when you think about it, these original stains really would have a certain je n’sais quo; what meal had the baby eaten, and what prompted him to throw it up? Was it the strained beef or the apple pie…
Here are a few examples of how it is used on the Web. These lines were extracted from the search results for Je n’sais quo, in the Google search engine: ‘Those movies which have a certain je n’sais quo,’ ‘that certain je n’sais quo adds to-,’ and ‘Some of the best a certain je n’sais quo-’
It seems to be desirable, and it may even be essential, to preface je n’sais quo with the word ‘certain.’ It seems that no-one is writing, for example, ‘it has je n’sais quo. Instead, everyone is writing, for example, ‘it has a certain je n’sais quo.’
Why is this?
I plan to find out.