I too am a big fan of Dragon Ball series (Dragon Ball, Dragon Ball Z and Dragon Ball GT). There are many web sites in the internet which provides all episodes in the series for free. But, many of them cannot be trusted as there may be a chance of virus infection. So, I suggest you to prefer YouTube. In YouTube, almost all of the episodes (definitely not all) are available in English. You can watch or download from YouTube. In addition to the episodes, movies related to the series are also available in YouTube. You can also prefer torrents. Recently, I downloaded all the 276 episodes of Dragon Ball Z and all the 64 episodes of Dragon Ball GT using torrents. And coming to the part of your question, "How to watch dragon ball z episodes in Hindi", it is almost impossible to get episodes in local languages like Hindi, Telugu (my mother tongue) etc,. Trust me as I searched dragon ball z episodes in Telugu for 2 years but could not find them.
Hope this answer helps.