In which order should one build up a multilingual site? I'm planning to convert (better rebuild) an existing site based on MediaWiki and a phpBB3 Forum to a shiny new Drupal based site. The MediaWiki is multilingual, though most content is in German.
My experience with Drupal is basic. I've done basic things with other CMSs and am familiar with PHP, MySQL and HTML.

A couple of weeks ago I set up a new Drupal 7.10 instance (locally).
My first try was migrating the existing MediaWiki content as articles into Drupal. I failed badly and could not find a way (on the web) realising this.
I ended up building the central and essential MediaWiki templates (which are very individually) as content templates. That worked pretty satisfying and I accepted to manually copy&amp;paste the content in a later step (has somebody a monkey for me?).

Then I tried migrating the existing phpBB into a Drupal-based forum via a module (failed) and finally agreed on bridging those. The modules for this are actively developed and I let some more time pass as to get them more mature.

Beside those two central topics of the desired Drupal site, multilingual content is essential as well. For the wiki-like content even more than for the message board. In case of the wiki content, the users will be creators of new and translators of existing content.

While trying different ways and different modules for all the functionality, I have to admit, that I got hopelessly lost.
I'm not asking for a specific set of modules, I should use. I would have to define all functionality of the desired Drupal site and that's not the core of this question.

My question is rather: In what order should I build up the site's functionality?
That is, after having set and defined the access and user groups, should I first concentrate on the multilingual backend and translation pipeline (e.g. modules for multilingual content and translation)?
Or should I first get the templates for the wiki-based content right and implement the multilingual thing atop of that?
Both is somehow closely interconnected and -- as mentioned above -- I got lost while finding my own way.

I hope, this question is not too broad and vague. If so, please let me know and I try even harder getting it more specific.
I would suggest starting with setting up the multilingual backend. Besides what Drupal core already offers you probably want to use the i18n module. Then you could move on to content migration. If there is no module that can do that for you I'd suggest writing a PHP script rather than manually copy/paste-ing (seeing as the amount of data seems to be fairly large). You could make use of either node_save() or drupal_form_submit() for this task. The first option is a quicker way of adding nodes while the second mimics more closely normal node creation by invoking additional hooks. There's a solid amount of information about using them to create nodes programmatically available on the web that you can make use of. At what point to work on appearance (templates and theming) shouldn't matter too much, but it's probably somewhat easier at later stages when you already have the content to work with.