How are you adding the category to that page?

Docs explain what's currently supported in Translate: https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration#Markup_examples (in short, all in same category or template wizardry that you have to find out yourself).

karin.jeppsson wrote:

It doesn´t matter if I put the </translate> tags above or after the category as this:

Mypage

<translate>
this is all text in my page....

[[MyCategory]]
</translate>

OR

<translate>
this is all text in my page....

</translate>

[[MyCategory]]

A copy of the page for the source language (in my case english)is still created like: Mypage/en. This means that in MyCategory I end up with 2 pages (for the source language, not a problem for other languages) one source page and one copy with suffix "/en". Why is a copy of the page for source language created? I am not going to translate it.
Hope you understand my problem.
//Karin

Yes, I know it works like that, it's also explained in the docs linked above.
What I don't understand is what categorization system/scheme you're using.

karin.jeppsson wrote:

Translation by adding language suffix: Category:Cars/fi
I have one category for each language.
Ex
Cars/sv
Cars/fi
Cars/ru

Again, when I've asked in the past I was told this isn't going to be supported (not even by examples in the documentation).
You have to do it on your own with templates, like [[mw:Template:Langcat]] (in use on translatewiki.net too).

Maybe in the future this will be rethought, but this is the situation.