It would be highly unusual if they matched orders from several currencies.
That is precisely what they do.
Is it really written anywhere on their page or another reliable source? This
However, since the USD volume is usually higher than other currencies, trades in USD have more chance to hit another order in USD.
I have understood like "if you trade in USD you have more chance to execute the order", not that "order with EUR can be matched by order in USD".
Well I'd like to think of myself as a reliable source.
https://support.mtgox.com/entries/20800336-multi-currency-tradingQ. Do you keep independent ask/bid tables for different currencies?
A. No, ask/bid tables for different currencies are not independent. All currencies are relative to whichever currency has the highest volume, which is based on said currencies current market price in bitcoin. Every trade is in one pool and in fact, not are not separate currency markets. This allows users the added benefit of trading in "the greater market" -in currencies they understand- while not limiting them to smaller currency markets.
For example, if a buy order for bitcoins is placed in EUR, the order can be executed against another user selling bitcoins in any currency and not necessarily only against another user selling bitcoins in EUR.