As far as I am concerned, MC does everything you asked for very well, and there is no need to use any other tool, except for identifying your music by AcoustID and adding extra tags from an external source. I use MusicBrainz Picard for that. MC can do it, but the database used (YADB) often isn't as complete as with MusicBrainz Picard. YADB is good when ripping CDs though, I find.
How you structure folders really doesn't matter in MC, except for backup or selective copying to other devices, or if you are going to use something other than MC to play your music, in which case you need to take into account the other player requirements.
For example, MC already has a [Language] field, and if you put "French" in there for all your French music, MC could easily show you just your French music and no other, using a View. It could also, for example, synchronise only your French music to an external device, such as an iPod. You
could structure your folders by [Language]/[Artist]/[Album] for example, but there would be no need.
Even if you structure your folders some preferred way, then change your mind later, MC is the best tool to move files around and restructure folders. Read this Wiki article:
https://wiki.jriver.com/index.php/Rename,_Move,_and_Copy_FilesAlso search around the forum for discussions on the functionality, often known as "RM&CF". It is very powerful functionality, and with great power, comes great responsibility, so be careful using it, and test on small groups of files before applying to your whole library.
So no, I don't think you should do anything on your HD before importing your music into MC. As long as you have decent tagging on your files, any inconsistency in folder structures can be fixed from within MC. If you don't have good tagging now, you may wish to use MusicBrainz Picard, or mp3tag, or some other tools to populate tags before importing, but honestly, I find it easier to work with MC checking what I need to fix, and then look up files that don't have tags using MusicBrainz Picard externally.
Note also; importing your music into MC does not move the files at all. It just indexes them. So if you import them all, look around and decide you want to improve things externally before continuing with MC, you can easily do that and then just start again with MC, importing into a new clean Library (database).
Enjoy the journey that is JRiver Media Center!