Regarding the Indonesian from English description...
The contributors claims that "After this course, you’ll also be understood by Indonesian and Malay speakers in East Timor, Malaysia and Brunei."
However, as a native Bahasa Malaysia/Melayu speaker speaking from experience, that sentence is not true at all. There is a lot of vocabulary differences between two langauges, like the most obvious would be Merry Christmas in Bahasa Malaysia/Melayu is "Selamat Hari Kristmas", while in Bahasa Indonesia is "Selamat Hari Natal"
Despite the grammar is more or less the same, the vocabulary difference can make speaker of Bahasa Indonesia unable to understand Bahasa Malaysia/Melayu.
So I wish the contributors of Indonesian delete that claim to prevent misunderstanding.
I'm not familiar with these two language variants; I only know something about the differences between British English and American English and Spanish Spanish and Latinamerican Spanish (which has variants of its own):
Vocabulary differences are not difficult at all. As shown in your sentences, you can guess the meaning of the sentence from the sentence as a whole: You wish something happy. The context will make it clear what the sentence is about: Christmas.
For the learner who learned Bahasa Indonesia, it's relatively easy to fill up this vocabulary gap: Once they've been told that "Kristmas" is "Natal" in Malaysia, they will simply replace that vocabulary, and that's it.
When someone travels to a country whose language they learned in a language course, there will be many vocabulary gaps even though the person learned vocabulary in the course, too. This is normal because the vocabulary of a language in its actual use is much more varied and complex than can be taught in any language course.
The problems resulting from this have to be solved by simply inserting the word that people actually use into the sentence one is speaking. This is something that a traveller or migrant learns on-site -- in any language.
Therefore, I think that the claim of the language course is adequate.
This is an interesting point and I understand your concerns. I can only speak of my own experience with these languages. I have travelled in Malaysia and Brunei using basic everyday Bahasa Indonesia when English was not understood. Most of the time, locals were able to understand the gist of what I was saying, vice versa. In Malaysia and Brunei I found the hardest thing was not differences in vocabulary but rather the different accents and pronunciation.
In a more recent interaction, I was speaking with an English person who had worked in Indonesia and thus was fluent in B Indonesia. Nearby was a Malaysian woman, and little did we know she was actually listening to us speak B Indonesia. At the end, she told us that she had understood everything we were talking about which was an amusing surprise.
As someone mentioned here, the difference between B Malaysia and B Indonesia as variants may be akin to Spain-Spanish and Latin American Spanish, in that they have the high grammar and vocabulary overlap. I would suggest that the difference between B Malaysia and B Indonesia is more like the differences between languages in the Romance family. For example, if you understand Spanish then you will have some understanding in French, 'a leg up', so to speak, because they have the similar roots in Latin, thus their grammar and most of the vocabulary also cross over. They are two languages with common roots. Likewise, B Malaysia and B Indonesia share similar roots in Malay. The differences are there, especially when you compare the vocabulary, spelling, accents and pronounciation. But IMHO they are similar enough that the speaker of one can understand the speaker of the other.