Presumably because if you don’t speak about things that you love, then you are doing them a disservice. Perhaps other people will speak wrongly about them, or the world will remain ignorant about them, or you will forget about them.
It’s actually the first part that is messy to analyse (the second is actually pretty crystal clear!). To me it looks like an archetypal example of those pretty, but fuzzy quotations that turn out to be somewhat vacuous when you try to make sense of them (i.e. to translate them).
Luckily, it means pretty much exactly what it says without calling for weird metaphors or strange meanings: a literal translation going Although to speak of things you love is to betray them, not to do so is no less a betrayal. is IMO perfectly accurate. In that regard, I don’t think posing it as a question about a foreign language is helpful. It’s really more a question of literary/philosophical analysis and falls well outside the scope of our site.