internationalzing my site to be able to use multilanguage

Please I tried to download polylang plugin on my website After download everything on my site stopped working
I consulted the polylang owner to investigate. This is what they sent to me.
(Take care that your theme must come with the corresponding .mo files (Polylang automatically downloads them when they are available for themes and plugins in this repository). If your theme is not internationalized yet, please refer to the codex or ask the theme author to internationalize it.)
Please I am send this if anybody will be able to help me on this?