Translators are very used to paying close attention to whether a Spanish sentence contains the word "no" or not, because it can obviously change the meaning from positive to negative. However, the subordinate conjunction hasta que often occurs with a superfluous no (the grammatical term for this is the "pleonastic no") that has no meaning. Thus, for example:
Hasta que no haya una recuperación económica, no aumentará la demanda de viviendas means "Until there is an economic recovery, the increase for housing will not increase" (obviously "until there is not an economic recovery" would make no sense).
Another example of a Spanish phrase containing a word that you must omit in translation is antes al contrario, which simply means "on the contrary, quite the opposite, far from it." The word antes here carries no meaning and does not appear in the translation.
On the other hand, there are a few Spanish phrases where a word has to be added in English translation. For example, en absoluto means "absolutely not" - so "not" has to be added.
In the sentence En mi vida he visto semejante cosa, the phrase en mi vida is like en absoluto and thus carries an implied negative, so the correct translation is "Never in my life have I seen such a thing" (even though you don't see the word "never" there in Spanish).