What is the hardest foreign language to learn?

Vulcan

Femalese for at least one week per Month

For an American it would have to be any language that English has no basis in.

I say Mandarin.

Mandarin

But all these are all very difficult. Duolingo is your friend for commons phrases and words if you're going to any countries where these languages are spoken.

My dick

I've heard Mandarin Chinese, because based on the inflection of your voice a word can mean entirely different things.

Japanese isn't that difficult except for the 3 alphabets. Hiragana actually makes sense. The letters represent the actual sounds unlike English where we use crazy shite like ph to make the f sound. Katakana is like hiragana but it's used for "foreign" words, things that were invented outside of Japan (yes they're racist and they don't care). Kanji is where it all gets fricked up because there are thousands of these symbols. And they use them all at the same time.

I've never understood why the entire world isn't taught sign language as children. It would minimize communication barriers throughout the world.

Whatever that language is that is spoken by the inner city democrats. That language is confusing.

When I was at the Defense Language Institute they had ratings for each language rated from 1 thru 5.
They stated that English was the only Cat5 language.
I was there in 1993 so this may have changed since then.

CAT5:
English

CAT4:
Korean (most difficult in this group because of all of the Chinese symbols required to read and write Korean)
Mandarin Chinese
Arabic
Russian

CAT3:
Vietnamese
Japanese
There are a few others...

Romance languages (Spanish, Italian, German, French) were CAT1