This needs more than just a doctype and some optimisation, the code needs a complete overhaul.
If you can, then start from scratch. Avoid table based layouts and validate on your way.
Cut down on all those flag images, they're too much.
Your taking on a big task, trying to maintain this thing, good luck.
Disclaimer. (1) Whilst I will help you sometimes, if I feel like it, and my advice in relation to your actual question will be of good quality: my posts are to be taken with a pinch of salt. I will be sarcastic, deploy irony and include obscure cultural references for my own amusement without warning.
(2) You will gain nothing from complaining, and if you try to argue with me then you will not win. No matter how noble your battle seems, I am still better than you, don't be an hero.