1. Yes, which is exactly what you WANT, especially if you've already been through one or more major updates. I'm not going to go through ALL of the reasons why, but one of the biggest reasons you want to do this is that they seem to make major changes to the driver frameworks during each new major update much like what we used to expect when updating, for example, from Windows 7 to 8.1, 8.1 to 10, etc.
This tends to create issues for a lot of people much like a change of platform, so a lot of us recommend doing a clean install every time there is a major update OR at least periodically, not going more than two or three major updates without doing a clean install. Obviously, if you have no issues after the upgrade at all then that's fine, but if you have any unusual behavior or issues that were not present previously I'd immediately look at that option if you don't want to just skip the middle man and go straight to a clean install.
Yes, you ALWAYS need to backup any important files. You should backup any and all important files EVERY time you do a major update, before changing hardware, and periodically (Like, at least bi-weekly) whether you are changing hardware or updating in any way anyhow. Otherwise, you have nobody to blame but yourself when something happens to the system, and it will, because it has for pretty much everybody at some point due to a borked installation or failed drive.
As far as languages go, that is strictly a property of your advanced settings. I'm pretty sure the is secondary language support in ALL versions of Windows 10 and if you current installation supports it, then any upgrade or clean install should support it as well.
The most important thing to do before every doing anything at all, is make sure your Windows license or digital entitlement is attached to a Microsoft account. That way there can be no question of whether or not it is valid after a clean install. You simply log into your Microsoft account or enter your old product key and poof, activated, even after a hardware change, unless you have an OEM version of Windows 10 and in many cases, even then.