There are nations that can learn English easily (Northern European countries for example) and nations that can’t learn English that easily (Japanese, Korean, Vietnamese, etc.)
Now we are going to talk about Vietnamese and why learning English is so difficult for them.
The biggest problem of Vietnamese learners of English is pronunciation. They go to school, learn hard, then when speaking English with foreigners they realize that nobody understands a word they are saying. But, interesting enough, when they speak English with other Vietnamese they can understand very well. There are two main reasons why this happen (I call them "problem no 1" and "problem no 2") and I will try to talk about each problem and provide a solution.
Vietnamese is a monosyllabic language. All the Vietnamese words are made of a single syllable. Because they are used to pronounce monosyllabic words, pronouncing English words is a quite difficult task. That’s why they pronounce first syllable of a word (maybe the second in some fortunate cases) and everything after that is left apart. That’s why the past tense of verbs and the “s” sound of the third person singular present simple and plural of nouns are also left apart.
The best example is the word “because”. It is pronounced /bɪˈkɔ/ intead of /bɪˈkɔz/.
Since Vietnamese are quite good when it comes to spelling, the teachers should exploit this and always emphasize that all English consonants must be pronounced. Although it is not 100% true, I would lie in this case. I did the same with my wife and it works very well. Now she tries to pronounce all the consonants and pronouncing difficult English words is not so difficult anymore. Practicing as much as possible and having that rule in mind seems to be a good way for improving pronunciation.
In Vietnamese language you are not going to see two consonant sounds next to each other but this is quite common in English. That’s why a simple word like “school” is pronounced /kul/ instead of /skul/.
This problem can be solved with the fist solution (pointing out that every consonant has to be pronounced) or similarly can be easily fixed through reading but there is so little reading in classroom. The foreign English teachers are always told that they must teach speaking and listening, not reading or something else. As many learners told me, when reading they don’t have to think about grammar (as in case of speaking) and they can focus on pronunciation.
I have done a test long time ago with my students. First I asked one of the students to read a short paragraph at normal speed just as he would read a book. I have recorded it and then I played it again pointing out all of the pronunciation mistakes. After that I asked the student to read again but this time it should be read slowly and focused on pronunciation.
The difference was huge; so much better pronunciation. After this stage is completed, the teacher can start doing the speaking activity. That’s why I always ask my students to write down everything they are going to say and read it at least once.
The teacher can also ask students to underline the part of the word where there are two consonants next to each other so when reading they can easily see where the hard part is and of course pay more attention to it.
There are many other reasons why English is difficult to pronounce for Vietnamese learners but solving these two problems means solving 90% of their pronunciation mistakes.