The answer to the question what is the hardest languages to learn depends on one’s own native language. Languages are not equal: some are more structurally, phonetically, and syntactically complex than others. For instance, for me a Moroccan Canadian whose native language is Arabic, learning the different Arabic dialects spoken in the Middle East is pretty easy. Most of these dialects descend from classical Arabic, which is the proto-language or the parental language.
For an English native speaker, Arabic is one of the hardest languages to learn. Why? Because Arabic, among other reasons, descends from a different language family (i.e, Afro-Asiatic) than English (i.e, Indo-European) which explains why Arabic script, letters, grammar, syntax, and intonation are all different.
If you ask a multilingual person (i.e., a person who speaks two or more languages) about their foreign language learning experiences they will certainly tell you that they found a certain language hard to learn compared to other languages they learned.
However, to attribute language difficulty solely to the differences in ancestral language decent is to miss the point and fail to understand the immeasurable complexity of human language.
As I was working on Best Languages to Learn, I had the chance to consult several reliable online resources that feature the hardest or most difficult languages for English speakers to learn. All of these lists unanimously feature Arabic and Mandarin among the hardest languages to learn.
As for Babbel, these are the 6 hardest languages for English speakers to learn:
For Rosetta Stone, their list of the hardest languages for English speakers to learn comprises the following languages:
For Busuu, these are are the 8 hardest languages for English speakers to learn: