TITLE: Mathematics in Other Languages
QUESTION [30 upvotes]: The intention of this community wiki is to aid in learning another language in the restricted topic of mathematics. It does matter whether one wants to learn another language for mathematics or use mathematics as a stepping stone into another language. This post focuses only on the written aspect of the language. 
For each language, there is a a small description, which includes its difficulty and pay-off, and a list of works to help learn it, which includes one or two grammar books and several works written in the language, which are accompanied by a description that includes their difficulty of reading. If it is a historical paper or it difficult to find in print, then a link to it will be provided if one is known. Since this is more about learning the language, works that are easier to read are preferred, although very famous works might also be mentioned, and, in the case of ancient languages, an exhaustive list list of all mathematical works in this language is ideal. Works that include a heavy amount of mathematics, but whose main topic is not mathematics but a related field, such as physics or astronomy, are sometimes acceptable, especially in the case when they are written in dead languages. 
Some General Tips:

Often, dictionaries and translators do not translate mathematical vocabulary well. A good way to go about this is to use Wikipedia by switching the language of the page.

REPLY [4 votes]: Portuguese
In the recent years, developing countries in the Southern Cone have gained an unprecedented importance in global mathematical research, spearheaded by Brazil's Instituto de Matemática Pura e Aplicada (IMPA) which recently hosted ICM 2018 and is home, among many others, to Artur Ávila, the first Latin American to receive the Fields Medal for his work in dynamical systems.
Being a romance language, there may be difficulties in learning Portuguese for the native English speaker, but in the case of Spanish speakers it is quite the opposite: both languages are similar enough to the point that Spanish and Portuguese speakers can understand each other symmetrically with little trouble (this is evident e.g. in tourism and there's even a related linguistic phenomenon named "portuñol").
Another Portuguese-speaking country is Portugal. Brazilian Portuguese and Portugal's Portuguese only have slight differences (e.g. words for verb tenses, and cultural differences in what constitutes a formal voice).
General Resources:

(I do not know of any introductory texts aimed at English speakers, sadly.)

Mathematical Resources:

One of IMPA's explicit aims is to build a specifically Brazilian mathematical literature, and thus many books regarding all sorts of subjects (from first year undergraduate mathematics up to cutting-edge research monographies) are available. They are distributed by the Sociedade Brasileira de Matemática (though shipping options don't seem to be available...).
Recordings of IMPA lectures, colloquiums and events are available at their YouTube channel and are an excellent resource to improve listening skills.