Because they share the same linguistic ancestry with each other, Indonesian and Malay can be understood by each other with relative ease. But they can't possibly be the same thing! Differentiating characteristics between the two include pronunciation, intonation, spelling, vocabulary, and phrases. Some terms used interchangeably across the two languages have such vastly different connotations that misunderstanding them might lead to awkward situations. To provide just two examples, the Malay term "budak" means "kid" but "slave" in Indonesian, and the Malay word "bual" means "conversation" but "brag" in Indonesian.
The consequences for language instruction are less humorous, although these disparities between Indonesian and Malay do provide for funny tales. A language instructor who knows just one language and has no background in the other cannot possibly inform their pupils of these important differences and may leave them vulnerable to embarrassing circumstances. Similarly, students who are fluent in one language but have no experience with the other may be in for a rude awakening if they believe they can do adequately in both.
Indonesians felt a strong urgency to establish Bahasa Indonesia as the official language of the country after their triumphant return to freedom from Dutch colonial control. There was a need to differentiate it from Malay, the common language of Malaysia and Brunei. As a result, Bahasa Indonesia developed its distinctive grammar and vocabulary over time. In contrast to the English pronunciation of the Roman alphabet, which is used by the Malay people, the Indonesians use the Dutch pronunciation. In addition, the "r" at the end of Indonesian nouns like "besar" and "tidur" is pronounced explicitly with a resonant trill, but in Malay it is quiet, as it is in English. For this very moment, it is possible to determine the speaker's nationality with absolute certainty.
Bahasa Indonesia also made an effort to incorporate various terms that are unique to Indonesia into its lexicon. As an example, Malay does not use "nggak" but rather "tak" for the negative. If a speaker uses the word "nggak" in his discourse, it is clear that he is actually from Indonesia and not Malaysia. Indonesian has absorbed numerous Dutch terms that are foreign to Malay speakers, such as "handuk," which means "towel" in Indonesian, and Dutch but "tual" in Malay, which is more closely related to English.
The lesson for teachers of Malay and Indonesian is that they should not believe they can effectively conduct instruction in any language simply because they are native speakers of the other language. Similarly, students shouldn't try to save time and effort by learning only one language, since doing so would mean missing out on the nuances that are specific to each tongue.