BANGKOK, Dec 30 (Reuters) - Most Southeast Asian stock markets hit multi-week highs on Wednesday as end-year portfolio buying lifted select shares in holiday-thinned trade, marginally trimming losses on the year for underperformers Thailand and Indonesia. Bangkok's key SET index ended 0.3 percent higher at 1,288.02, the highest close since Dec. 17, but fell 14 percent on the year, among Southeast Asia's laggards. Domestic institutions led buyers as valuations of Thai shares have come do
