It works about as good for (simple) French as for English in my experience, but is absolute garbage for Russian—because, I suspect, of all the noun morphology. But, to be fair, the moment Google started becoming comparable to Yandex for Russian was the moment it started to randomly substituting terms in my English searches, like switching “Fréchet space” for “Banach space” in complex queries (they are both things in functional analysis, good job Google, but they are not the same). So looks like it’s either the heavy-handed clustering we hate about the current Google, or difficult linguistic work and tuning for every language in the world.
(Is there a way to stop DDG from ignoring the quotes in my searches? At this point I sometimes get better results from Gigablast than from Google or DDG when searching for a quote.)
Turns out making a search engine is a whole lot of work, I genuinely hope they keep working on it!