I'm going to give you a heads up on this slytherin, because what you need is not just help, you need a lot of time put into this, and sadly time is not always free

I'm going to list this step by step, and you would do well to follow the steps, and then you will have a great looking forum with no errors, but it takes a bit of time, and no something someone can do for you in 5 minutes
STEP 1:
Always backup any files that you intend to change. Therefore, if you have uploaded replacement language files from version 8.05, then you will corrupt your 9.51 version

because those files are out of date
STEP 2:
The Dutch language files for version 9.51, as far as I know, do not exist. They have not been created as far as I am aware. Therefore, the files that you need to alter are in the langauge_files FOLDER:
There are a TOTAL of 8 different ASP files that have hundreds of lines on some of them, that need to be altered. Example is below:

And then you have to painstakingly go through each and ever file and as an english person, try and translate into a language you don't even know

And still working


You could do this yourself. Its only a question of spending hours sat at the computer, and changing english to dutch

STEP 3:
Skins
Skins again take time to sort out. They are not as simple and as straight-forward as people assume, and again, it would be great if everything in life was free, but it is not. There is no good trying to import skin files from OLDER web wiz versions, as the whole structure of the new software will not even match up