The stocks app uses the Yahoo Finance codes.
To use stocks from other exchanges, you add that exchange's suffix to the stock ID. This page lists the current exchanges:
http://finance.yahoo.com/exchanges
Example:
France Telecom
Stock ID is FTE in NYSE
Stock ID is FTE.PA in Paris
To track an index you can find them here
http://finance.yahoo.com/intlindices?e=europe
Example:
The London Financial Times Stock Exchange top 100 is ^FTSE
To track exchange rates go here
http://finance.yahoo.com/currency
Example:
Select to convert 1 US dollar to UK (British) pounds, click Convert and the result provides the ID (symbol) USDGBP=X