Vietnamese and Khmer, two languages of SE Asia, are quite hard for an English speaker to learn for a variety of reasons. We take a look at those two languages here.
Vietnamese is hard to learn because to an outsider, the tones seem hard to tell apart. Therefore, foreigners often make themselves difficult to understand by not getting the tone precisely correct. It also has “creaky-voiced” tones, which are very hard for foreigners to get a grasp on. Vietnamese grammar is fairly simple, and reading Vietnamese is pretty easy once you figure out the tone marks. Words are short as in Chinese. However, the simple grammar is relative, as you can have 25 or more forms just for I, the 1st person singular pronoun.
Vietnamese gets a 5 rating, extremely hard of all.
Khmer has a reputation for being hard to learn. I understand that it has one of the most complex honorifics systems of any language on Earth. Over a dozen different words mean to carry depending on what one is carrying. There are several different words for slave depending on who owned the slave and what the slave did. There are 28-30 different vowels, including sets of long and short vowels and long and short diphthongs. The vowel system is so complicated that there isn’t even agreement on exactly what it looks like. Khmer learners, especially speakers of IE languages, often have a hard time producing or even distinguishing these vowels.
Speaking it is not so bad, but reading and writing it is difficult. For instance, you can put up to five different symbols together in one complex symbol. The orthographic script is even worse in that sense than the Thai script. There are actually rules to this mess, but no one seems to know what they are.
Khmer gets a 4 rating, very hard.