You will need a power supply for your phone or tablet that uses European voltages. This sort of universal charger is usually labelled 110 - 240 VAC, 50 - 60 Hz. For reference, European voltage is typically 220-230 volts AC (VAC), at 50 Hz. North American voltage is usually 110-120 VAC at 60 Hz. Thus, a charger labelled 110 - 240 VAC, and 50 - 60 Hz will usually work.
Do avoid cheap Chinese-made chargers. There have been ample reports of scorched and "fried" mobile devices, and fires, as a result of using some of these. As a rule, I will only purchase an off-brand (non-Apple for example) charger from a known major supplier, like Belkin, Griffin, or Hama (in Europe)
For European use, the plug-end must be the two-round prong Shucko type plug, common in all of Europe (except the UK). Plug adapters are inexpensive and readily available worldwide. I use them and give them away. So I usually buy them a dozen or so at a time from:
(to convert two flat prongs to two-round Euro spec prongs)
http://www.220voltageappliance.com/...und-pin-accepts-polarized-plug-also-wmu5.html
(or to convert two round prongs to two flat prongs (my solution):
http://www.220voltageappliance.com/pack-of-4-european-to-american-flat-pinwmf7.html
Here, you can buy adapters, going either way, for only USD 2.99 for a pack of 4. That is USD 0.75 for one adapter. You cannot buy them, one-off, that cheap anywhere. If you are buying several units of 4 each, the postage is not too bad, especially when you consider what it would cost you to buy even ONE adapter in an airport gadget shop. I regularly see single plug adapters sold for €5 to €10 in airport shops. When my supply runs low at home, I just buy a dozen (3 x the 4-pack price). At the unit price I pay for them, I give them to friends, relatives, or other pilgrims I meet that need one, as I usually carry spares in my rucksack.
I regularly travel with Apple devices (iPhone and / or iPad) using a Lightning plug, and other electronic devices that require a micro-USB plug to charge. These micro-USB devices include a Canon digital camera and battery-powered, Bose ear-fit, noise-reducing earphones for flying.
My personal charging solution, and I travel to Europe regularly, is an Apple brand, Zippo-lighter shaped and sized Shucko plug tipped iPhone power supply that uses international (European) current. I use this is with combination charge / synch cable that I obtained from Griffin, the multinational accessory maker, that RETRACTS, and has BOTH a Lightning and Micro-USB tip attached. There is a standard USB connector on the other end.
I specifically settled on the Apple iPhone charger as it is flat and stores easier than the US sugar-cube shaped Apple adapter, or just about any other after-market adapter. My sole complaint is that Apple do not make a similarly shaped, higher amperage charger for the iPad. The Apple- Zippo charger will charge the iPad, albeit much, much slower.
Note, if your smart phone requires the same charging specification as the Apple branded charger provides, it DOES represents a small, easily-toted alternative to a larger, more square charger. You might consider it, if only from the esthetic viewpoint.
The Apple Euro-spec power charger, a US flat-bladed plug adapter, and the combination retractable cable all ride very flatly, in a small 5' x 4' LokSak ziplock plastic pouch that rides in one of my cargo pockets. This pouch is completely waterproof. It NEVER leaves my person. If I am wearing my cargo hiking pants, the charge kit is in one pocket...period. I usually get a few years and a couple of Caminos from one pouch.
These LokSak ziplock bags come in packages of 2 for USD 7.99 directly from LokSak. See:
https://loksak.myshopify.com/collections/aloksak/5x4 You can also get them from LokSak in a combination package of several different sized LokSak bags. This might be a good idea for someone who wants a variety of sizes, like for carrying their national passport and pilgrim credential. The 4 x 7 inch size works great for this. I use one of these to carry my documents completely protected in the other cargo pocket when on Camino. I might get soaking wet or fall into soft gummy mud, and I do and did this past April, but the electronics or documents in the LokSak bags are very well protected.
You can easily buy the Apple iPhone adapter for about €25 in any shop across Europe that sells Apple accessories. I have bought these in FNAC shops, Carrefour, and in El Corte Ingles in Spain. You can get them in almost any mobile phone shop that sells Apple accessories. Airport gadget shops also sell them.
I use a reverse plug adapter solution. That is a two FLAT plug adapter to convert the round prongs on the Apple charger to two, flat blades for North American use. This eliminates the problem of losing an adapter while traveling overseas.
For inflight charging, the airline provided power outlets will usually accept EITHER flat or round plugs...I checked yesterday on an American Airlines domestic flight from Washington, DC to my home in south Florida. Also, I verified this was the case when I returned from Spain two weeks ago on United Airlines.
Stated another way, when traveling, I make my "native" plug configuration the Euro-standard, with a separate converter for North American use. To me, IMHO, this is the smarter way to go to avoid not having what you need when you most need it, which will usually be where a plug adapter is least available. Murphy's Law rules...
I hope this helps.