By multilingual, do you mean have all the content in both languages or do you mean have some posts in one language and some in the other. If it is the latter, I would only recommend it if the majority of your targeted audience knows both languages. From an SEO standpoint, both situations that I talked about shouldn't cause problems- in fact, it will be more content, which is always good (as long as it is unique and not copied from some other site.)
some of the translation plugins will do a bunch of languages as well. I haven't personally tried them but it's not a bad idea to at least give it a test run.
Here is a good translator plugin for WP. I use it on mine, and it's great. You have about 35 different languages to translate to. http://wordpress.org/extend/plugins/global-translator/