Additional information, such as what device, what model, what operating system, and what particular steps have been made to download the language, would make it easier for us to assist you.
To do so, please join the site so you can reply in this thread with the additional information requested. See this link for instructions on how to join Windows Central.
You haven't told us what version of w10m you are on. If you are on the latest 'preview' there is an issue with language packs. As mentioned above, join us and these bits of info will be at your finger tips