English as a lingua franca for Europe

Pronunciation difficulties

The sounds indicated by the letters th, voiced interdental fricative and voiceless interdental fricative, aren't found in other European languages with the exception of Spanish, Greek and Icelandic. The French replace it with the sounds 's' and 'z'; Scandinavians with the sounds 't' (or 'f') and 'd'. English as a lingua franca would choose one of these sounds to standardise on it. The reason this is curious is that, unlike vowels or liquid consonants, tongue placement is specific: place the tip of the tongue more forward and form the sound between the tongue and the upper teeth. It is recognised that vowels and the American or German 'r', on the other hand, are much more difficult to learn because the sound is made by a relative placement of the tongue.
The letter l in English also corresponds to two different sounds, lateral alveolar approximant and velarized lateral alveolar approximant—respectively, the first 'l' in the word, 'little', and the second. Polish has them too, but marks the dark l with a line through it, and over time has diminished the sound to that of a 'w'. Germans have difficulty with the dark l, which does not exist in their language.

In most of the other Germanic languages, like German, Dutch, and Swedish, consonants at the ends of words are never voiced, and so native speakers of those languages tend to not voice consonants at the end of words in English, hence mug and muck, and bat and bad are pronounced alike to them, and they generate confusion by pronouncing the present tense of build the same as the past tense, built. This latter extends to their writing.

Phoneticists note that besides the difference in vowel quality, there is also a difference in length between the vowel sounds in the words bit and beat. Speakers of languages that don't have vowel pairs with this distinction, such as Italian and Spanish, often have difficulty with this distinction, although the distinction does occur in Germanic languages.
The most obvious difficulty is the large number of vowel sounds in the English language, each one of which has to be learned by listening and training tongue placement.

Intonation difficulties
English is a language with stressed syllables, both unmarked in writing and capable of changing the meaning of words and even sentences. Although words without the usual stress can be understood by native speakers, changes in meaning of sentences spoken by them ("I thought SHE was supposed to wash the pan" vs. "I thought she was supposed to wash the PAN") are often entirely missed.

Punctuation differences
The British use their punctuation rather similar to the French, but not entirely. Though persons can learn another language, they often slip back to the punctuation of their native one. This is most obvious in the so-called temporary allowances for writing in the English language to not use the same punctuation of metric measurements as those in other countries. Germans do not re-convert their decimal points from a comma back to a dot when writing in English, nor do they use the comma as a separator of groups of three digits. The French do not count like other peoples, so their telephone numbers are set in pairs of digits, and this has extended already to many British telephone numbers.

Vocabulary difficulties
Non-english speakers, especially the Japanese, sometimes take English words and modify them for concepts that they think appropriate, but which will not be comprehended by native speakers. The French have a rationally prescribed vocabulary, so they often cannot notice that many concepts second nature to them mean something different, if anything at all, in English. The primary example are false friends such as the French and German words actuel (aktuell) and eventuel (eventuell), which in English don't mean actual and eventual but rather current and possible.
It is expected that a standardised ELFE would declare many of these neologisms normative, so as to have native speakers have to use them in the same way, too, when communicating to Europeans.

Spelling difficulties
The British spell many words just as the French do; a few words Americans instead spell as the Germans do. But the French and Germans spell many similar words differently and will use these in their writing.
Last but not least, any ISO for the English language might very well bring about spelling rationalisation, of all things the most difficult about the English language for any use on any continent.
(See also the recent experiences with German_spelling_reform.)

Annex
EuroEnglish
The European Commission has just announced an agreement whereby English will be the official language of the EU rather than German, which was the other possibility. As part of the negotiations, Her Majesty's Government conceded that English spelling had some room for improvement and has accepted a 5 year phase-in plan that would be known as "EuroEnglish": --

In the first year, "s" will replace the soft "c".. Sertainly, this will make the sivil sevants jump with joy. The hard "c" will be dropped in favor of the "k". This should klear up konfusion and keyboards kan have one less letter.

There will be growing publik enthusiasm in the sekond year, when the troublesome "ph" will be replaced with the "f". This will make words like "fotograf" 20% shorter.

In the 3rd year, publik akseptanse of the new spelling kan be expekted to reach the stage where more komplikated changes are possible. Governments will enkorage the removal of double letters, which have always ben a deterent to akurate speling. Also, al wil agre that the horible mes of the silent "e"'s in the language is disgraceful, and they should go away.

By the 4th yar, peopl wil be reseptiv to steps such as replasing "th" with "z" and "w" with "v". During ze fifz year, ze unesesary "o" kan be dropd from vords kontaning "ou" and similar changes vud of kors be aplid to ozer kombinations of leters.

After zis fifz yer, ve vil hav a reli sensibl riten styl. Zer vil be no mor trubls or difikultis and evrivun vil find it ezi tu understand ech ozer.

ZE DREM VIL FINALI KUM TRU!!