You see the image of mobile phone with usb cable when you try to go to fastboot mode ( power button + volume " - " ): in this case you have the bootloader locked.
I use these weekly roms and I haven't any problem.
If you don't want a developer rom, you can use a stable rom:
https://www.androidfilehost.com/?fid=24591000424958681
First of all you should request the bootloader unlocking directly to xiaomi.
Go to:
http://en.miui.com/unlock/
Click on "unlock now" and follow the instructions.
After one week, you will receive an sms to confirm the unlock of bootloader.
I think you have a problem with xiaomi drivers.
Download Mi Flash tool or Mi Phone to correctly install the drivers.
If this doesn't work, try with another PC.
You're confusing. The root permissions are one thing, the bootloader unlocking is quite another.
Miunlock is a software for unlocking the bootloader.
Have you requested the bootloader unlock to xiaomi?
If you want the french language, you should flash the xiaomi.eu rom.
If your device has the bootloader locked, first of all you should unlock it.
http://en.miui.com/thread-202290-1-1.html