search engine integration with wordpress internal vs external

We are planning to build a private multilingual wiki (need user name and password to login)site for our researching team members, with about 15k pages, it is very dynamic (content are constantly updated), we are looking for a better way to improve the search function in our private wiki. We would like the full text search function can return a more accurate and relevant result.
We did some homework and went through this article:

so we are looking for suggestions about the best options for our wordpress based wiki.

Plugin (internal) or external search engine? Any recommendation?

Here are some questions that we have:
1. Crawler vs API
Correct me if I am wrong, crawler actually is not a good option for a private site, API connection sounds handle password protected website better. does API manner can handle our needs?

2. API Privacy & security
Per our understanding, the indexing data will be kept on service party’s server, and we want keep the content private, I understand it depends on service’s party’s privacy policy, but we would like to know, in a common sense, will they keep customers indexing data/database in a secure and private way? or they will share with other for their purpose.
another thing, working with external search engine, how to keep our data secure and private? Including the connections with external party.
3. Multilanguage
Many research content is mixed with western and eastern language, will their search engine support that?
4. Integration
Is it easy to integrate external search engine service with our wiki site?