Who designed these pages and this translation process?

> Why is it not right in the menu tree?

It's right there in the menu, click "Translation tool", then click "edit" on the wanted message group. Two click, it is too difficult? You can also use Special:LanguageStats/fi.

> What to translate

As I wrote above, You can browse easily. Or browse this. Pretty colorful and handy statistics. The red ones really begs for attention.

> terminology

I think it's the responsibility of the translators building the terminology database. We (hungarian translators) built one here.

> information on how parameters or single words are used is insufficient

Yes, You are right. Although it is not twn's fault, but the developers of mediawiki/extensions. Any user can add translator hints, although few MediaWiki developers do it. If you're really stuck with a message, please do not translate it, but ask for help on its meaning here, and someone will add the documentation for the message... It's done the wiki way.

> you use Google translate

It only helps. If one copy the google translation character by character, he/she shuld be kicked in the ass. But google translation often helps, and individual words can be copied instead of typing a lot.