The "Market Value" section of the Account Window, within TWS, is a real-time reflection of the account's currency balances.  Clients can see their balance in any currency at that moment by referring to the Market Value section. If the Market Value section is not reflecting all currency positions you think you hold, look to the right of the words "Market Value" and click on the green plus sign ( + ). This will expand the Market Value section fully. You can see the cash balance being held in each currency, along with the Net Liquidation Value of those same currencies. The Net Liquidation Value will reflect the current cash balance, along with any interest payable/receivable and the value of any securities held in the account which are denominated in that currency. 

The "FX Portfolio" section of the Account Window, within TWS, is utilized primarily by clients who actively trade and speculate in the Forex market. They use this FX Portfolio to keep track of all currency crosses they have open at a given time.  When they close these positions, they reset their FX portfolio to zero. The FX portfolio reflects net trades done in a currency pair over a given period of time. If a client does not reset the FX portfolio after each trade, the result is that it does not show an accurate reflection of the account's currency positions/balances. Rather it is a net total of trades done in that currency pair, which could include trades, conversions, etc. It will not be an accurate representation of the account's currency positions or balances.  In order to reset the FX Portfolio, you can right-click on any Forex pair line. A small window appears, and in that window select Adjust Position or Average price. Another small box will appear, and in that box change the position and price to 0 (zero). Click OK, and the FX Portfolio will reset for that particular currency pair.  

To reiterate the above point, the Market Value section of your Account Window will show your real-time balances/positions in any currency.