English is as voracious a language as any other, I imagine, and its tendency to gobble up words and expressions from other languages is surely encouraged by the fact that in many predominantly English-speaking parts of the world (such as North America) the language is constantly being enriched by contributions from immigrants who speak other languages and who add anglicized versions of words and sayings from their native languages to the local or regional English mix.
Consequently I was not greatly surprise to discover that Wolfgang Mieder, A Dictionary of American Proverbs (1992) includes this proverb, collected in Ontario:
Don't throw away your old shoes before you get new ones.
In a Google Books search, most matches for the expression (like this one from 1893) note that it is originally Dutch, or are by Dutch authors writing in or translated into English (like this one from a 1970 article on quantum physics—one of two instances in the same article), or are datelined from the Netherlands (like this one from The Rotarian [Chicago, April 1935], datelined Utrecht). But this example from Roger Welsch, Mister, You Got Yourself a Horse: Tales of Old-Time Horse Trading (University of Nebraska Press, 1981), appears to be straight-down-the-fairway U.S. English:
Luther wasn't a bad sort but a man who would throw away his old shoes before he had any assurance that he could hang onto his new ones was none too promising as a credit risk. Still, I thought, his foolishness was the means of making me near a thousand dollars.
Likewise, "Senex," a letter writer to the editor of the [Melbourne, Victoria] Argus (August 16, 1880), uses this form of the expression in a column titled "Religion in State Schools":
If one cobbler hid supplied him with a pair of ill fitting shoes, he says that he would apply to some other to make him a better. But that implies the existence of another cobbler whose shoes had been proved to be better. I ask him to name the cobbler and to produce the shoes. To cast off the old shoes before we have found our cobbler, would be to go barefoot, and I don't wish our children to go morally barefoot.
So is "Don't throw away your old shoes before you get new ones" really an English proverb? Probably not, unless your standards for what constitutes an English proverb are extremely easy to satisfy (as Mieder's seem to be). But is it an expression that has appeared multiple times in English over a period of many years, and does it make sense as a proverbial saying when read in English, and do at least some English speakers treat it as an aphorism? Yes, yes, yes.
Denying that something is a part of English just because it didn't start in English is so dubious a position that I doubt anyone here would endorse it. And just because I may be unfamiliar with an expression doesn't mean that it isn't part of everyday English speech in some part of the English-speaking world. These considerations lead me to think that answers suggesting English versions of proverbs from other languages may well be useful to people who ask questions such as What's a good idiom or saying to say "don't leave your current job before getting another"?
In fact, as here, the suggested expression may turn out to have appeared in numerous English-language publications in the past and may be a legitimate English expression. But even if it had never been used by native English speakers, that doesn't mean that the figurative expression from Dutch, when translated into English, wouldn't be worth considering as an effective way to express the idea in English.
For these reasons, I oppose deleting such suggestions, unless they appear without any supporting documentation or explanation. (And of course, our standard practice in instances where an answer lacks adequate support is to ask for such content, rather than to immediately delete the answer.) Not only are these suggestions from other languages potentially useful to the question asker, but sometimes they turn out to be established in English already.