Tests are written for various goals: integration testing, to prevent regressions, and in the same effort to prevent regressions to protect mission critical / business logic code. If all those nuances are captured by good tests, you arguably have "100%" test coverage, you don't need to test every single line of code ever written to have 100% test coverage in my eyes. But then when you go to translate your project to a new language, you port the tests first, then test against those tests.

This is my personal belief on this anyway.