Sounds good in principle, but learning to speak all SE Asian languages would require you to be some kind of linguistic genius and probably half a lifetime of devotion. Speaking only one of the languages from the region, say Thai or Burmese or Vietnamese, wouldn't help much in neighboring countries since each language in the region is not mutually intelligible with only limited overlapping...so such an approach would mean limiting your engagement to one country.

In parts of northern Laos and northern Myanmar, no attempts are made by Chinese settlers to learn the local language - everything (signboards, menus etc.) is in Chinese and locals [Laotians and Burmese] who can't speak this foreign language [Chinese] are left out.

Also, I don't think English should be forgotten - despite various levels of English fluency in the region, English is still the only global lingua franca and the global language of business. English is the only language you can successfully use in all SE Asian countries. While it's great to know Thai, Lao, Burmese etc. it isn't realistic unless you are living in those countries. I have also noticed that just like with English, Chinese learners of these SE Asian languages really struggle, and when encountering a local that speaks good English, the conversation will usually inevitably switch to English since the local will assume the other party can't speak their language well enough. It's only once fluency is achieved that this is overcome.

My recommendation would be for both Chinese and other foreigners interested in investing/doing business with the region to know how to speak English fluently, followed by becoming proficient in at least the basics of their host countries' language to at least show some interest and respect. Apart from those interested in becoming translators however, I personally think time and money is better spent gaining technical skills and then applying some language skills on the side - not the other way round.

I've done that and I'm doing quite well. I am an engineer that has worked in Vietnam and Thailand and I speak Thai and some Lao with an almost native accent (and can read and write both languages) - something that is of enormous benefit to me, but I have achieved this as a side passion rather than as my main job. Still, I barely speak Vietnamese and don't have the energy or time to work on it - in any case, doing business isn't difficult as most educated people there speak English anyway and I have a very good friend who helps me so it's all good. Ditto for Cambodia and Myanmar.