Mussen staan er, zijn er. Mussen blinken uit in aanwezig zijn. Op post zijn, op tijd. Liefst te vroeg, beter nooit te laat. Weer of geen weer, mussen vliegen van nest naar nest. Mussen zijn zwervers.
Sparrows are around, they exist. Sparrows excel in being present. Being on their post, on time. Rather too early, preferably never too late. Come rain or shine, sparrows fly from one nest to another. Sparrows roam around.
About the title, this is exactly the same structure as in dutch, a good reason to keep it IMO.
I looked it up in a dictionary, and "to be threatened with extinction" was the most appropriate way to translate "met uitsterven bedreigd zijn".
"Weer of geen weer" (litt. weather or no weather) is a proverb, the Spanish equivalent would be "haga frÃo o calor" or "haga el tiempo que haga". I found "come rain or shine" as a good equivalent in a dictionary as well
If you really think these both sound weird (I might be using a cheap dictionary :P), we could always ask Kafetzou?