Hi Yau
I am french native, I stay and live in Paris
Well I have to say it is true that we make fun of Belgian people
But it is all stereotypes... french are likely to do so with any foreigners
... such as english, swiss, germans... and so on!!
About the spoken and written language, there are very few differences. But we do understand each other at 100% . So if you go to Belgium to learn french you will totaly be able to speak with french native
You may have a very light accent and very few different sayings but nothing serious.
So my advice is: going to Belgium is not a bad choice at all
Just keep going on your plans
I am glad you'll learn french