There are two possible reasons for this:

1. Article has been created in certain language, and not translated into other - then article is displayed only on that particular language setting, and missing on others when you switch language to different one. Solution: ask author to translate it in other languages, or if he does not want to translate it, then he should at least switch language-settings of that article to "language neutral"!
2. Article has been created with "language neutral" setting (no matter what language has been actually used). Then it is not possible to translate it, and the same version is always displayed, no matter how you switch language. Solution: ask author to switch settings from "language neutral" to actual language used in article, and translate it.

In any case, learning foreign languages might help you too! :-)