While London, Boston and Washington happen to be cities I will pass through on my next holiday, there is a better reason for bringing them up here: they are very often pronounced incorrectly by non-native speakers, including people on television, radio and even English teachers. As I will tell you below, there are many other examples of the same mistake. In principle, we have to distinguish two cases. First, the correct pronunciation of these names in English, and second, the correct pronunciation in German. Whereas London has a distinct German pronunciation, see here and listen here, Boston and Washington are, according to the Duden, to be pronounced in the original way, see here (around the 1min mark) and here. However, it appears that very few people are following this rule, and instead a modified, German pronunciation of Boston and Washington has been quite firmly established.
Let me explain the typical mistake by starting with London. The correct English pronunciation of London is [‘lʌndən], listen to it here. The important point is that the second o is pronounced as ə, the so-called Schwa sound. The mistake that is often made is to drop the second o, leading to the incorrect pronunciation [‘lʌndn], listen here (#1). Other examples where the Schwa is often dropped incorrectly are the cities Washington and Boston, which are correctly pronounced as [‘bɒstən] and [wɒʃɪŋtən]. You can listen to the correct and incorrect pronunciations here. Once you are aware of this mistake, you will find yourself hearing it all over the (German) news and in films, etc. Two good examples from the German news can be found here and here. From personal experience, I think that the reason for using this pronunciation is that it makes these names sound less English and more German, and hence easier to fit into a German conversation. While this seems acceptable and may even become the standard German pronunciation of these words some day, it is important to remember the correct English pronunciation.
The reason why especially German speakers make this mistake is that the rules for when the Schwa is dropped are different in German and English; in German, it can be done much more often than in English. For example, the German word landen is pronounced as [landn], listen here, and mischen is pronounced as [ˈmɪʃn], identical to the English word mission. I found a nice discussion of this issue in a German publication by Sonja Biersack, see here, on page 83. In the case of London, the so-called Schwa-Elision also leads to the incorrect German pronunciation [lɒnd’n], whereas the correct pronunciation is [lɒndɒn], listen here (#2). In fact, I still remember one of my English teachers telling us that the correct pronunciation of London (supposedly used by Londoners) was [‘lʌndn], which is of course wrong. Interestingly, when comparing the English and German versions of TV spots by the company Rimmel (“Get the London look!”), you will notice that in the English commercial London is pronounced as [‘lʌndən] whereas in the German commercial it is [‘lʌndn]. Make sure you also listen how The Clash pronounce London.
Another good example is the town Dresden, which has the same name in English and German. However, while German speakers usually pronounce it as [dresdn], non-native speakers are likely to use [dresdən]. The latter pronunciation is also the way you should pronounce Dresden when speaking English in order to make native speakers recognize the name of the town.
Interestingly, it seems to be quite hard to avoid this mistake for non-native speakers. For example, I have heard Boston being mispronounced as [bɒstn] by an otherwise almost perfect non-native speaker who has been living in Boston for many years.
Let me finish with some more examples. Similar to London, the Schwa in
Simpson (including Homer’s family, see here)
is not dropped in English. The same is true for chicken and Rock ‘n’ roll, which are almost always pronounced incorrectly by German speakers. Although the German pronunciation here is very different from the English pronunciation, the n (short for and) in Rock ‘n’ roll is pronounced as in English (the Schwa sound is not dropped). For chicken, the [ɪn] used in English is replaced by [en] in German, see here. However, [ˈtʃɪkn] is not correct. Finally, the most complete source for the pronunciation of geographical names seems to be the Longman Pronunciation Dictionary.