Is it possible to read an attached document, written in Nepali, in Gmail, without downloading it? And if yes, how?
Gmail has inline Message Translation- a bar should appear at the top an email opened in a foreign language and you can select the source (Nepali in your case) and target languages (if auto-detect doesn't produce your desired combination).
If the translation bar doesn't appear you can click the more down arrow (next to the reply button) and select Translate message
Whether or not it can translate the attachment depends on the document format- if you open and view the document (inside of chrome) some formats should provide you this same translate options as those that exist for the messages themselves.