, but my idea is to write some post in Spanish and put them in a Category called "Spanish". I don't want those to show in the Homepage

There is only one main page for posts in a blog and all posts will display on the main blog page. There is no way to exclude posts from the main posts (blog) page. The front page of the blog by default will display your posts (not pages) in reverse chronological order, with the most recently published post on top. If you do not want all the posts to show on the front page, then you can create a static front page “Welcome“ for your site and a “Blog” page for posts.

I'm sorry we don't have any bilingual themes. The only themes we can use are those found here > Appearance > Themes
We can set only one interface language for our blog here > Settings > General
You can have 2 blogs in 2 different languages, of course, but we have don't support automatic translations between them. You'd need to write a blog post in the two languages and then post to each blog (or post the two versions of the entry on the same blog, make each language category, create a custom menu and add the categories pages to it).
http://en.support.wordpress.com/menus/#creating-a-custom-menu
http://en.support.wordpress.com/menus/#adding-category-pages-to-your-menu

Choice 1 static front page
Choice 2 running page for all posts on front page

Which do you choose? It’s one or the other. If you choose Choice 1 static front page then you have to create two pages as I said above. After you do that you go to > Settings > Reading and make the designation change and click “save changes”.

Please let me know.