I would vote for Dutch (the language of the Netherlands) . It's fairly close to English, as Dutch is in some respects part-way between German and English. And Richard S is right about the Frisian dialects of Northern Europe. Around a hundred years ago, I've been told, fishermen from the far east coast of England and fishermen from Northern Germany could actually understand each others' dialects. However, most dialects have now disappeared, and today there is no language which is mutually intelligible with English. Or in other words, there's no other language whose speakers can understand English if they haven't learnt it. And to answer your question, Natalia, no, French isn't related to English. The structure of English has more in common with the ancestor language of German and Dutch. We do have thousands of words in common with French, though, and thousands more which are similar. But these are all 'adopted' words which came into the language after 1066, or at a later stage through Latin.

From Wikipedia:(http://en.wikipedia.org/wiki/English_language#Classification_and_related_languages) The English language belongs to the Anglo-Frisian sub-group of the West Germanic branch of the Germanic languages, a member of the Indo-European languages. Modern English is the direct descendant of Middle English, itself a direct descendant of Old English, a descendant of the Proto-Germanic language. Typical of most Germanic languages, English is characterised by the use of modal verbs, the division of verbs into strong and weak classes, and common sound shifts from Proto-Indo-European known as Grimm's law. The closest living relatives of English (besides the English languages and English-based creole languages) are the Frisian languages of the southern fringes of the North Sea in the Netherlands, Germany, and Denmark. After Frisian come those Germanic languages that are more distantly related: the non-Anglo-Frisian West Germanic languages (Dutch, Afrikaans, Low German, High German, Yiddish), and the North Germanic languages (Swedish, Danish, Norwegian, Icelandic, and Faroese). None of the Continental Germanic languages is mutually intelligible with English, owing in part to divergences in lexis, syntax, semantics, and phonology, and to the isolation afforded to English by the British Isles, although some, such as Dutch, do show strong affinities with English, especially to its earlier stages. Isolation has allowed English (as well as Icelandic and Faroese) to develop independently of the Continental Germanic languages and their influences.

I think that French is close language to English. In school I learned French and when I started to learn English, it was easy. English and French have a lot in common: articles, words (f.e. "attention" is the same word in both languages, only differs pronunciation), word order in sentences etc.

Honestly, after thinking a lot about this question, I think the answer is that NO language is close to English. English has Germanic roots, but ancient German, not modern German. English has a lot of influence from French, but very little in common today other than similar-sounding words. Because English is a mix of several different languages, with influences from many different places, and centuries of developing on its own, it's hard to say that English is similar to any single other language. I will say that I've studied French and German, and I find that the grammar for German is quite comfortable for me at the basic level, and basic French vocabulary is easy for me as well because it has many similarities with English.

I would agree with Johnny. You would probably need to ask a specialist linguist and they might give you a complex answer dealing with different aspects of the language. If you ignore the Norman French influence on the language, there is a good argument for saying that Dutch is the the most similar language. It could be interesting to ask Dutch, Danish and German people the same question.