We've decided to rank the order from easiest languages to learn to the hardest.
- Spanish. Speaking: Very Easy. Grammar: Very Easy.
- Italian. Speaking: Easy. Grammar: Easy.
- French. Speaking: Moderate. Grammar: Moderate.
- Portuguese. Speaking: Moderate.
- German. Speaking: Difficult.
- Hindi. Speaking: Moderate.
- Mandarin. Speaking: Difficult.
Considering this, which is the closest language to English?
As Gustav Meeuwenfiaster noted in a comment to Diego Hernandez's answer, Frisian is the closest to English. Dutch and Afrikaans come next. Danish, Norwegian and Swedish are also quite close. These are all Germanic languages and share a common (long extinct) ancestor.
What is the most difficult language for a native English speaker to learn?
The most difficult languages — Arabic, Chinese, Korean and Japanese — require, on average, 1.69 years (88 weeks), or 2,200 class hours, to reach speaking and reading proficiency. Each of the four "hard" languages is difficult for its own reasons.
What is the most similar language to English?
The languages I found to be grammatically closest to English that I have studied, though (in order of similarity, first is most similar) are Afrikaans, Dutch, the Scandinavian languages (Swedish, Norwegian, Danish) and German mostly because of grammar (they are all Germanic languages).