As Slugster mentioned, selfie sticks are not, inherently, designed for close quarter combat. However, if you really want your stick to be effective in self-defence situations, there are a few factors to consider when choosing your "weapon" : techniques (length), materials, transportability and legality.
Techniques (length) :
From your question, it seems that you are familiar with a good selection of weapons. Since you seem to be more familiar with swords and sticks, my suggestion for you would be to pick a selfie stick that's around 2-3 feet long when retracted. This allows you to use most escrima or short sword techniques that you already know (it would be my personal pick for similar reasons, having learned the okinawan tonbo in the past).
However, some people might be more familiar with self-defence weapons such as the kubotan. In this case, consider getting a small telescopic stick that about 6-7 inches long when retracted. At this point, it's basically a kubotan, and you should be able to use any appropriate techniques with it.
Some companies also make very long selfie sticks, going up to 12-18 feet when extended. Once retracted, these sticks are usually about 6 feet long, which means that you could probably wield one like a staff or bo until it breaks.
Materials :
From what I can see, there are three main materials used in the fabrication of selfie sticks : fibreglass, aluminium and carbon fibre. Carbon fibre would be the hardest, but might shatter if the impact is significant enough since most selfie sticks are hollow and quite thin (because they don't have to hold significant weight). Aluminium will bend the most, and is likely to stay bent after any strong impact. Fibreglass is somewhat in between, but is probably closer to carbon fibre than aluminium in terms of resistance (will flex more than carbon fibre, akin to aluminium, but is more likely to shatter).
Some companies, like Youngblood, make fibreglass coated aluminium selfie sticks. We're talking about ~200-250$ selfie sticks here, but that combo is actually pretty kick ass. Just like concrete reinforced with rebar, the aluminium core will help the stick flex more instead of shattering, and the fibreglass exterior should be hard enough that it would take many serious strikes to permanently bend your impromptu weapon.
In any case, you should aim for a telescopic selfie stick. A selfie stick that is made of a single tube is mostly empty inside, and is thus a lot less resistant. If you use a telescopic stick and leave it retracted, the 2-3 sections, when nested one within the others, will make a surprisingly resistant stick. To be fair, it's really likely to break if you use it as a weapon. But if its 2-3 layers thick, you should have time to finish your fight before having to change weapon.
Transportability :
Even if you actually want to use it as a selfie stick, only put your phone on it if you're taking pictures. The phone itself is a target for thieves and, while I'd say your life is worth more than your phone, why even risk your phone if you don't have to? That being said, you'll want something that's easily carried, and inconspicuous enough that people will let you carry it wherever it is that you're going.
This is why the 6 feet long stick isn't really interesting, despite doubling as a really great impromptu staff. A palm-sized stick can be carried on your belt with a strap/clip or even inside your pockets, making it the most portable option. If you want more reach, however, my personal choice would be a 2 feet long stick that can extend to 4 or 6 feet. While it can't be carried inside a pocket anymore, it can still easily be strapped to the side of any backpack (or even carried inside it, though it makes retrieving it a chore in an emergency situation), or just carried in your hand without generating much suspicion.
Legality :
Depending on where you're going, laws can vary greatly where weapons are concerned. I can't speak for Europe, but I can give some example for Canada, since I happen to be familiar with the local weapon laws. As Sardathrion mentioned in a comment, some places, like Canada, actually consider intent to determine if something is a weapon or not. For instance, I could carry pretty much any kind of knife, as long as it's for a utilitarian purposes (skinning game, utility knife, box-cutter, etc.). The instant that I say it's for my protection, even if I never intend to use it offensively, its possession becomes illegal.
That being said, if I get mugged and happen to have a knife or a selfie stick on me, which I am carrying for legitimate purposes, I would not hesitate to use it in self-defence. Justifying minimal appropriate use of force to the cops might be a little harder for the knife than for the selfie stick, but if I can prove that I'm carrying this item for purposes others than self-defence, it should be legal as long as the danger I was facing warranted it. From the link provided by Sardathrion, the UK seems to also take the appropriateness of the force used by the defender into account in cases of self-defence, and it is likely that similar laws are in effect throughout the European Union.
If anyone asks, you're carrying your selfie stick to take cool & touristy selfies. And if you happen to use it in self-defence, do not exaggerate in your use of it as a weapon and do your best to keep the fight from escalating. Stop fighting as soon as you believe it is safe to do so for you, your loved ones, but also for your aggressor.
