Indeed, it might be a problem in countries where there is rarely any native literature. Because in such countries English competes against other transnationial languages (Spanish in Hispanic America, Arabic in Islamic world, French and Swahili in parts of Africa, Russian in some post-Soviet states, …), not much against native languages of local population. Situation is not very different from the one in France, Germany, Spain etc.
When to encourage? Of course, only after acquiring sufficient ability to read in English through some effort lo learn the language. There are some English textbooks with mathematics-related texts, possibly even “Simple English Wikipedia”… but here is not English educators’ site, indeed. Starting to learn a language from reading professional-level scientific texts wouldn’t be a good idea. IMHO any person with some pretence to be educated must be able to read in more than one language (in two with advanced level, or in one with advanced and in yet two with intermediate, maybe). Given English isn’t a difficult language, students shouldn’t necessarily be exposed to it very early. Maybe there should be more prioritized foreign languages. Either due to regional circumstances (see the first paragraph), or just because learning well-structured languages (which English doesn’t belong to) may improve thinking skills. But if a student is aimed to a world-class research, then almost certainly s/he will ultimately learn English to extent of understanding scientific texts.