Fairly common all over the world, in the UK the commentators are fairly neutral when talking about a Premier League game, but when it comes to the Champions League or an international game then it's the pride of England vs those dirty foreigners it's ridiculous.Never a better example than last years Champions League when it was basically Barcelona vs England (well it was pretty clear the winner would be either us, Liverpool, Man Utd, Chelsea, or Arsenal) so they spent most of the build up talking about how superior the English clubs were and the Premier League style was. Then when it actually came time to play an English team in the semis, in the first leg Chelsea played the most disgusting stay back hack and burn cancer football and were being lauded by he commentators as wonderful. Barcelona should have been awarded a stonewall penalty when Henry was pulled down in the box and the commentators said something like maybe Chelsea got a away with one there but it's hard to say. It finished 0-0 and post match the panel of 'experts' were raving about how Chelsea pulled off an incredible defensive display, completely ignoring the penalty and the fact that Chelsea committed 30 fouls and received 1 single yellow card, whereas Barcelona committed 6 fouls and received 3 yellow cards.
Then in the the second leg every time a Chelsea player got into the area they commentators were calling dead set penalty, and when Barcelona scored the last minute winner one of the commentators uttered the words "Oh no". Post match they identified 5 different stonewall penalties as evidence that Chelsea got robbed. In reality there was 1 undeniable penalty and a bunch of 50/50 calls, they ignored that Barcelona had a player sent off wrongly when a Chelsea player tripped over his own feet, and no mention of the penalty that should have been given in the first leg. Chelsea were robbed according to the English commentary team, but now there was no way that Barcelona would beat the mighty English champions Manchester United who were going to make history by winning the CL twice in a row for the first time.
Although you haven't truly heard biased until you've heard an Irish commentary team during a Ireland rugby match.