During my intercultural contacts I noticed that English is the lingua franca for people from different countries. When I speak with foreigners I often ask them which languages they speak. Very often they speak French, and as I speak French fluently, I would rather speak French than English. Sometimes these foreigners even admit they speak better French than English, so I propose to speak French and they agree to do so. However, when I meet the same person later, automatically we start speaking English, even if we both speak French better. This is because we are kind of trained to speak English with people who do not speak our language, unless we do speak their language (fluently). Most Flemish people will speak French to French people and no English. Some cultures appreciate it very much if you speak their language and will automatically talk to you in their language if they know you are able to speak it (e.g. Spanish people prefer talking Spanish to me as I'm able to speak it).

Sometimes it is also important to communicate clearly in English and to use simple words, as not all people speak or understand English very well. Sometimes you can notice from a person's accent from which country he is. Some words or syllables are difficult to pronounce in English.
Spanish people tend to say: "Espanish" for Spanish, "Datch" for Dutch (as there is no such sound in Spanish)
French people have besides their typical French 'r' also difficulties pronouncing the word "the" and just pronounce it as "ze".
Sometimes you can guess which expression is used in a language, e.g. once a Spanish girl told me "I had reason". She meant that "I was right" but as she did not remember this expression in English she just translated literally from Spanish (Tienes razón).

The link below shows you a funny example of what I meant above.