Add/Fix mulitlanguage-support for typo3 website

I have an existing typo3-website in english. Most of the content already exists in two more languages and is already inserted into the CMS a the right places. I do need some adjustments to the typo3-template. What I need is the option on each page, to switch to a different language version, also after changing the language, the links should point to the correct language version of the linked (internal) sites. So your job would be to insert the "language picker", check if the mulilanguage-support of typo3 is set up properly, check the template for "harcoded" links that need to be adjusted to mainatin the chosen language, and to make sure, the content appears in the correct language, once chosen... If you've done multilingual typo3-sites that should be no big deal for you. Looking forward to you offers.