I am a danish. Is it possible for me to apply 90days duration L visa to China?

You can try to apply for a L visa with 90 days stay duration per entry, but the final decision is up to the visa officer. Nobody can sure you can get it. If you can provide sufficient documents to prove that you really need to stay in China for 90 days, it is possible for you to get it.

hello I am a Korean. now, I'm traveling from Indonesia to China and will visit in Bangkok to get a visa for China. I'd like to get a 6month multiple visa for China in Bangkok. is it possible to get it there? if it's impossible, so which country is possible to get it?

It is very hard for you to get a 6 months multiple entry visa in Bangkok because you are just a temporary visitor in Thailand. It is possible for you to get the multiple entry visa in your own country.