This post will focus on how hard it is to learn the Malay and Indonesian languages for an English language speaker. As you can see below, these are two of the easiest languages to learn for anyone, really.
Bahasa Indonesia is an easy languages to learn. For one thing, the grammar is dead simple. There are only a handful of prefixes, only two of which might be seen as inflectional. There are also several suffixes. Verbs are not marked for tense at all. And the sound system of these languages, in common with Austronesian in general, is one of the simplest on Earth, with only two dozen phonemes. Bahasa Indonesia has few homonyms, homophones, homographs, heteronyms, etc. Words in general have only one meaning.
Though the orthography is not completely phonetic, it only has a small number of exceptions. The orthography, nevertheless, is one of the easiest on Earth to use.
The system for converting words into either nouns or verbs is regular. To make a plural, you simply repeat a word, so instead of saying pencils, you say pencil pencil.
Bahasa Indonesia gets a 1.5 rating, extremely easy to learn.
Malay is only easy if you learn the standard spoken form or one of the creoles. Learning the literary language is quite a bit more difficult.
Malaysian get a 2 rating for moderately easy.