The form an for the indefinite article is used before a spoken vowel sound regardless of how the written word is spelt. If you say an otel when speaking (which is now often regarded as distinctly old-fashioned), then it may be appropriate for you to write an hotel. But most people say hotel with a sounded h and should write a hotel.
By contrast, words such as Honour, heir or hour in which the h sound is dropped are written with an. Americans who drop the h in herb may also prefer to write an herb. But in standard British pronunciation the h is sounded and a herb is therefore correct in writing.
Because European is said with an initial y sound which counts as a consonantal sound in English speech, It is said (and written) with a not an. An abbreviation such as M.P. which is pronounced em pea begins with a spoken vowel and so it is an M.P.