Which South East Asian language?

Bahasa Indonesia (Indonesian) has more speakers than any other Southeast Asian nation. Also, it is similar to Malay, so you would be able to communicate with Malaysians. Indonesian is considered to be the easiest Asian language for English speakers to learn, because it uses the same alphabet and the structure is more similar than many.

If those sound like good qualities, then perhaps Indonesian is the best for you.

Chinese, japaneses, and koreans are east asians. Vietnamese, cambodians, laotians, filipinos, thais, and malaysians are south east asians. I can easily tell the difference between east asians and southeast asians, but not between their different nationalities.

None.

The languages most often used will be either Chinese or English

thats to influence of the Association of Southeast Asian Nations (ASEAN).

it's upto the situation of yours if you want to communicate for business or job then i don't know which one but if it's just a learning pleasure then malay and indonesian are easy to learn

Best would be the one that you are planning to use. There is a lot of trade taking place with Vietnam.

Learn Tagalog. You just say the words like it's spelled. like "panget". That means ugly in English, and u jst say it like it is spelled.

Catalog?

Indonesian is really easy but in the end it comes to what your interested in.

... x o x o !

Best is relative ... a matter of opinion.

if it is for a job...Mandarian Chinese...is good to know...i personally would never learn it