Master thesis: should I say that some results are a novelty in the field?
This is a common situation, since it really isn't possible to be absolutely certain that nobody has done this work before. What I find people usually do is:


Do the best literature search you can. As for references in other languages, in many fields it's  common that papers written in, say, French, will have an abstract in English, so you'd have a chance of finding them even if you can't read French. But if you find a French paper whose abstract suggests that it may contain similar results to yours, then it is your duty to find a way to read it (look for a translation, ask a colleague to translate, learn some French yourself if necessary) and make sure.
Once you are satisfied you have done a diligent search, write something like "this result appears to be new" or "as far as we know, this result has not been obtained previously". 
If it later turns out that your result was already known, be prepared to acknowledge that the other author had priority, and to be able to show that although you did a reasonably diligent search, you did not find the prior paper, and that your result was truly obtained independently (not plagiarized).