Latino sine flexione is a variant of Latin created by Peano in 1903. As far as I know it was used in scientific literature but since forgotten.
I found this site and a few discussions on Duolingo but not a single speaker.
A quite complete grammar can be found here.
Is this wonderful project dead?
It seems the original project evolved into what is now called Interlingua. The Oxford Essential Dictionary of Foreign Terms in English defines Latino Sine Flexione as:
The language Interlingua, in which nouns are taken from the ablative case of Latin nouns.
Meanwhile, the Encyclopedia Britannica states:
Interlingua, also called Latino Sine Flexione, simplified form of Latin intended for use as an international second language. Interlingua was originally developed in 1903 by the Italian mathematician Giuseppe Peano, but lack of clarity as to what parts of Latin were to be retained and what were to be discarded led to numerous “dialects” of Interlingua, confusion, and its dying out among enthusiasts. In the late 1940s and early 1950s, the linguist Alexander Gode, with the sponsorship of the International Auxiliary Language Association, reformulated and revived Interlingua and promoted its use in the international scientific community. As reformulated, Interlingua’s grammar is not much more complex than that of Esperanto; it has only one form for nouns (taken from the Latin ablative case), no gender, no case, plurals in -s, one form for adjectives with no noun-adjective agreement, one definite article, and verbs with no inflection for person or number. Abstracts and summaries are published in Interlingua by several international scientific journals, but in general the language has not been widely adopted.
Wikipedia, on the entry of Interlingua, says:
Today, interest in Interlingua has expanded from the scientific community to the general public. Individuals, governments, and private companies use Interlingua for learning and instruction, travel, online publishing, and communication across language barriers. Interlingua is promoted internationally by the Union Mundial pro Interlingua. Periodicals and books are produced by many national organizations, such as the Societate American pro Interlingua, the Svenska Sällskapet för Interlingua, and the Union Brazilian pro Interlingua. ...
Interlingua has active speakers on all continents, especially in South America and in Eastern and Northern Europe, most notably Scandinavia; also in Russia and Ukraine. There are copious Interlingua web pages, including editions of Wikipedia and Wiktionary, and a number of periodicals, including Panorama in Interlingua from the Union Mundial pro Interlingua (UMI) and magazines of the national societies allied with it. There are several active mailing lists, and Interlingua is also in use in certain Usenet newsgroups, particularly in the europa.* hierarchy. Interlingua is presented on CDs, radio, and television.
Interlingua is taught in many high schools and universities, sometimes as a means of teaching other languages quickly, presenting interlinguistics, or introducing the international vocabulary. The University of Granada in Spain, for example, offers an Interlingua course in collaboration with the Centro de Formación Continua.
Every two years, the UMI organizes an international conference in a different country. In the year between, the Scandinavian Interlingua societies co-organize a conference in Sweden. National organizations such as the Union Brazilian pro Interlingua also organize regular conferences.
Out of curiosity, yet there seems to be no Unix OS, Mac OS, nor Windows OS supporting Interlingua.
The IALA's language "Interlingua" seems to have been influenced by, but not identical to Peano's Latino sine flexione, which was also called "Interlingua".
The Wikipedia article on Interlingua (the IALA language), which luchonacho's answer links to, says
In its early years, IALA concerned itself with three tasks: finding other organizations around the world with similar goals; building a library of books about languages and interlinguistics; and comparing extant IALs, including Esperanto, Esperanto II, Ido, Peano’s Interlingua (Latino sine flexione), Novial, and Interlingue (Occidental).
Originally, the association had not set out to create its own language. Its goal was to identify which auxiliary language already available was best suited for international communication, and how to promote it most effectively. However, after ten years of research, more and more members of IALA concluded that none of the existing interlanguages were up to the task. By 1937, the members had made the decision to create a new language, to the surprise of the world's interlanguage community.
The vocabulary and grammar of Interlingua were first presented in 1951, when IALA published the finalized Interlingua Grammar and the 27,000-word Interlingua–English Dictionary (IED). In 1954, IALA published an introductory manual entitled Interlingua a Prime Vista ("Interlingua at First Sight").
Interlingua as presented by the IALA is very close to Peano’s Interlingua (Latino sine flexione), both in its grammar and especially in its vocabulary. Accordingly, the very name "Interlingua" was kept, yet a distinct abbreviation was adopted: IA instead of IL.
Here are some differences that I can find based on the Wikipedia articles on Peano's Interlingua and IALA Interlingua:
Minor pronunciation differences: Peano apparently was in favor of giving C and T the values [k] and [t] in all positions, while Interlingua seems to use the value [ts] or [s] for C before e, i, y and for T before i followed by another vowel. Both of these spelling-to-pronunciation rules exist as variant styles of pronouncing Latin itself.
Possessives seem to be formed differently: Wiktionary gives LSF "nostro" and "tuo" vs. IA "nostre" and "tu"
Certain function words: IA seems to have a definite article "le" that is not present in Peano's LSF. IA seems to use "a" in place of LSF "ad".
