Sometime we install language file (.mo file)properly but it can't convert entire web language. If you are an expert in po edit then this is not an advice for you. But if anyone cannot po edit then this advice is for him.

search for the last best compitable version of wordpress with the available .po file, then install that version of wordpress, then install .po language file. Now you can upgrade wordpress.
.mo file location: http://codex.wordpress.org/WordPress_in_Your_Language

You can find easily all version of wordpress including the latest at wordpress.org.