In the Renaissance era as it came to a close, many gentlemen wore breeches just like those, but most did not have the over fly cover of the buttons.
The legs either buttoned at the just below the knee or were tied.
I have a pattern used by the St. Michael's (Military Guild at Southern Renaissance Pleasure Faire) for the pants they use and it is virtually identical to what is shown in the picture of the Barbossa pants.
The thing about all that gathering in the waistband, is unless you are somewhat thin, it will most certainly make you look overly large in the hip/leg area.
When I've made breeches like that for people (myself included) I try not to gather too much material into the waistband to help create a smoother appearance. I'm also real fond of elastic waistbands (oh the horror as other costumers lament out there...) so it's a whole lot more comfortable and besides virtually no one sees the waistband depending on how the rest of your garb covers it.
The latest pair of pants I made are much longer, so if I were wearing boots I could tuck them into the boot tops. I also used black grograin ribbon for the ties instead of buttons.
You all have to remember, not all pirates were in the Carribean. In that area you would probably be wearing lightweight materials as opposed to the English coast where the weather is much cooler.
Also, fashions were handed down from one person to another. So it could infact be true that Dutch, German and other countries would have clothing from there in GoP.