When you hire a car in, say the USA, do you drive according to Australian road rules, or American ones? Obviously you must obey the local road rules.
Aviation is no different. You must abide by the LOCAL regulations when you operate in the airspace of another country - so for your overseas operation you must source the local documents as part of the cost/effort of operating. Most of them are available free online, however.
The European requirement (
EU-OPS 1.290 (c)) reads, in part:
(c) An operator must select at least one destination alternate for each IFR flight unless:
1. both:
(i) the duration of the planned flight from take-off to landing or, in the event of in-flight re-planning in accordance with OPS 1.255(d), the remaining flying time to destination does not exceed six hours, and
(ii) two separate runways (see OPS 1.192) are available and usable at the destination aerodrome and the appropriate weather reports or forecasts for the destination aerodrome, or any combination thereof, indicate that for the period from one hour before until one hour after the expected time of arrival at the destination aerodrome, the ceiling will be at least 2 000 ft or circling height + 500 ft, whichever is greater, and the visibility will be at least 5 km; or
2. the destination aerodrome is isolated.
(d) An operator must select two destination alternate aerodromes when:
1. the appropriate weather reports or forecasts for the destination aerodrome, or any combination thereof, indicate that during a period commencing one hour before and ending one hour after the estimated time of arrival, the weather conditions will be below the applicable planning minima (see OPS 1.297(b)); or
2. no meteorological information is available.
Note: "two alternates" means you must have a choice of two alternates available arriving at your destination - effectively you must plan fuel for the second closest (i.e.
not fuel for an "alternate for the alternate")
Some pilots think that if the destination has an ILS, weather below the non precision approach minima requires an alternate and if it only has non precision approaches weather below circling minima requires an alternate.
According to the rules, above, if your destination has weather above the minima for the best available approach - you plan ONE alternate.
If your destination has weather below the landing minima, you plan for TWO alternates (ie. not the closest, but the next closest). The alternates planned must have weather above "the second best approach" - if you are CAT3, that means CAT1 minima, if you are CAT1 (or only CAT1 is available), that means the VOR or NDB minima etc.
If the flight is less than 6 hours (basically local European) AND the destination has multiple runways (not crossing in such a way that both would be closed for an incident) then you can go without an alternate but with increased reserves (an extra 15 minutes @ 1500'). Most European ports are so close that carrying alternate fuel isn't limiting in any effective way. The only city I even contemplate not carrying an alternate is Madrid (in the centre of Spain, with every other airport situated around the coast it has the longest diversion distances.)