Our system limits the number of plural forms to four. Though, Arabic language requires six plural forms from what our users told us:
The current plural forms formula in launchpad is "n == 1 ? 0 : n == 2 ? 1 : 2" which I haven't seen used any where before, and is wrong, and I believe it caused corruption of Gnome translations since upstream PO files were using different formula.
Arabic has more complicated plural forms. The formula that we (at Arabeyes.org) agreed on is "nplurals=6; plural=n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n%100>=3 && n%100<=10 ? 3 : n%100>=11 && n%100<=99 ? 4 : 5;".
( https://answers.launchpad.net/rosetta/+question/23951 )
As a first step in supporting this, we need to extend our database schema to allow up to 6 forms. After that, we can change the code to support the extra forms. That change is bug 189890.
Fixed in RF 5644