[HansTWN]

Of course there are comparatively difficult languages. To give you an example: I know a lot of expats who have been to Indonesia for 6 months, and every single one of them speaks the language almost fluently. The same people now live in Vietnam and after 3-5 years just one (out of 10) has progressed beyond "Thank you", "a coffee with milk, please", etc. Only one of them can hold a very simple conversation.

The reason is that they can't get the pronunciation right to the point where they can be understood by native speakers. Apart from the 6 tones there are 3 different U's, a number of strange double vowels, and D's, T's, G's, Ng's, and other consonants of differing "hardness". So after a few months of enthusiastic learning, trying their skills in conversations with native speakers and being faced with "Huh??" every time they just gave up. Fortunately I am not easily embarrassed and I have the unfair advantage of speaking Mandarin (which is closely related) so I can hold a decent conversation even though I spend very little time there.

In some other languages it is extremely easy to be understood (take English, it is hard to imagine someone's pronunciation being so bad that you absolutely cannot understand a word he or she says), but very difficult to get the grammar right. But, of course, every language can be learned, you just need a lot more effort to learn the more difficult ones.