Saya membeli celana is: I buy pants....The correct translate is: Saya sedang membeli celana.. Continuous...
Indonesian is much more flexible than English, so this translation is not incorrect. Arguably yours is more appropriate though.
How did this identify has singular or plural? Why most be "s" at the back of pant
"Pants" is always plural in English. In modern English we don't tend to refer to anything as a "pant" (homonyms notwithstanding), and the item of clothing is always referred to as "A pair of pants"