The verdict of the Malaysian High Court is certainly wrong. It has nothing to do with Islamic teachings. According to report, a lower court of Malaysia had rightly allowed the use of the word Allah. But, in a later verdict, the High Court overturned the 2009 ruling of the lower court.
The word "Allah" is not the monopoly of Muslims. According to history, this word was prevalent among the idol worshipers of Mecca. The Quran itself certifies it. In this regard you can refer to this Quranic verse: "If you ask them who it is that has created the heavens and the earth and subjugated the sun and the moon, they will say, 'Allah.' How then are they turned away?" (29:61) If early Muslims were right in adopting this word from the pagans of the pre-Islamic period, then Christians of Malaysia are also justified in using this word.