From the huge amount of resources available to learn a language, I like the most the following two:
https://www.livelingua.com/fsi-language-courses.php
An improved "copy" of the Foreign Services Institute Language Courses, these are very complete courses that provide good practice and specially grammar documentation for learning.
Combining that with https://www.duolingo.com/, which provides a good interactive learning experience (without grammar), can give a well rounded learning experience for the available languages.
