Hilmy answered this question brilliantly. And if I might add: "Semua" is a more oft used word in general conversation. So you would't go wrong using the word 99% of the time (in spoken malay). "Segala" is pretty formal and is used more in writing, as in newspapers and novels.

From: http://bmcorner.blogspot.com/feeds/posts/default?orderby=updated here is the extracted text about 'segala' The correct use of 'segala' 13 September 2009 0:12 The numerical word (kata bilangan) 'segala' is now used for non-human beings. In the olden texts, instances of 'segala' being used for human beings are very common. However, modern usage does not allow 'segala' to be used for man. Look at the following examples: 1 Segala anak Encik Mazlan pandai belajar. (wrong) 2 Semua anak Encik Mazlan pandai belajar. (correct) [All Encik Mazlan's sons can study well.] 3 Segala rancangannya dapat dilaksanakan dengan berjaya. (correct) [All his plans was carried out successfully.] and from: http://malayjournal2.wordpress.com/ here is the extracted text about the difference: 6) Is segala and semua interchageable? Segala is all (generic): the universe or the set Semua is all (specific): the references