I recently discover new mathematical wealth in non-English texts. Sadly, the two language that I speak are not German, Russian, Chinese, French, or languages that belong to the world leading math research nations.
I want to learn these specifically for mathematics with NO emphasis on speaking, listening, basically NO conversation in general, just math - which would make the process more tedious.
What is the best way to achieve this goal? Would using a dictionary to read an article a good way to learn? But every language has some structure behind it and it may be impossible to figure out what certain things mean without that knowledge.
When I was a junior math major at a small midwestern (US) liberal arts college, my instructor chose a book in French for the text. He brushed away complaints that none of us knew French by saying that the math would be so hard that we'd hardly notice the inconvenience of learning French. He was correct on both counts.
That is my introduction to agreeing with @JeanMarie that you might do well to start with French. There are a lot of cognate words and French mathematicians seem to specialize in simple, elegant presentations. (With French, it does help to have someone around who knows the pronunciation, because it makes reading easier. The rules are fairly simple, and almost correct is good enough.)
Learning to read mathematics in any foreign language is made simpler by several almost-facts: almost everything is in the present tense and the third person, and you can begin by choosing books and articles on familiar topics so that you almost know what the words amongst the equations must mean.
One recommendation as you begin is that it may help to start with books written by people who were writing in what was to them a foreign language. Copy editors will have fixed the mistakes and the vocabulary will tend to be utilitarian. A Pole writing French, a Russian writing German, etc. Specifically, Casimir Kuratowski's Topologie and A. N. Kolmogorov's foundational tract on probability theory in German.
Another recommendation is to be absolutely fearless. No one will know about the initial mistakes you make. When writing my PhD thesis I needed to use a Russian journal article during the summer when all the Russian speakers I knew were on vacation. I knew no Russian, not even the alphabet. It took me almost half an hour to look up the first word, but then I (almost) knew the alphabet. I encountered a couple of places where it mattered whether the text was saying 'A of B' or 'B of A', but a quick look at a grammar text took care of that. Fortunately, the article was only 3.5 pages long, mostly equations. Three days, and I got what I needed.
I have no recommendations on getting started with Chinese.