It sounds like you're looking for one with a dictionary so you can look up a word you're not sure of, or don't know, right when you see it.
There are many that allow for dictionary lookups on the ereader itself. Some readers are better at it than others, of course. I don't know which ones are good at it, though, as it's not a feature I really need, so I don't try to keep track of readers in that regard.
One thing I'm not sure of is using a dictionary for a different language than the one you're reading. I would think that it wouldn't work well. For instance, if you see a word, you'd want to look up to find out what it means. But, since the definition would be in english, you could read the definition, and say "ok, I've got the definition... But, I don't know the words in the definition either"
There may be combination dictionaries, where the definition is done in multiple languages; you could look up an english word, and it would show you the definition in both english and another language. If a dictionary like that exists, and your native language is included, it seems like that would be great for you. I don't know if it exists or not, and will let others who have more experience answer that.
In short, I don't have a good suggestion for you like "Buy <xyz product>, it will do what you want and be the best thing you've ever bought."
But, hopefully this will give you some ideas to research.
(side note, there are lots of pc-based software packages out there that help you learn languages; depending on your needs, those might be better suited for you)