"What's the problem if a Christian Mass is performed in Arabic? "
If I went to a Catholic church on Easter expecting to hear a mass on the most important day of the calendar year without a for warning that this Easter the mass would be said in Arabic. I would be extremely upset.
Note: This is a parish that does masses in more than one language on a regular basis:
Sunday
Saturday, 5:00 p.m. (Sunday Vigil)
Sunday, 8:00 a.m. Portuguese/Brazilian
Sunday, 10:00 a.m. English
Sunday, 11:45 a.m. Arabic & English
Sunday, 8:00 p.m. English
Nowadays, it's not uncommom for urban parishes to offer masses in more than one language. It is important to check the mass schedule first. Nothing unusual.
Would you have been equally upset to find the service being conducted in Chinese? German? Russian? Polish? Vietnamese? Swahili? Because I can assure you, somewhere in the United States, there was a Catholic Easter Mass celebrated in every single one of those languages.
The language doesn't matter. It never has (except for Latin - and even then...). It's the form, the ceremony or ritual, and the *reasons* behind it that matter.
Christianity needs all the converts and believers it can get at this point in history. You want to turn people away from the Church because they might not understand English?
In addition, I suspect that, if it follows the model of most churches in the US, the times and languages of the services are not only posted in the bulletin, but on the signboard out front.