Searching for English translations of foreign language books and articles should be approached first by era.
1. Translations (generally excerpts) from the classical period may be found in one of the ‘Source Books in the History of the Sciences’
2. Translations of 19th century journal articles may be found in the Royal Society Catalogue of Scientific Papers, which was issued for the years: 1800-1863(v.1-6); 1864-1873(v.7-8); 1874-1883(v.9-11); 1800-1883 supplement(v.12); 1884-1900(v.13-19). The ‘Royal Society Catalogue’ is the definitive listing of scientific papers from this period, and many times the same paper will be published in the author’s original language and also in an English language journal.
3. Extensive English language abstracts of foreign language articles will be found in the Journal of the Chemical Society Abstracts, which were published as the even numbered volumes from v.34-128(1878-1925) and were preceded by the abstract sections in The Quarterly Journal of the Chemical Society of London, v.1-14(1847-1861) http://pubs.rsc.org/en/journals/journalissues/qj and the Journal of the Chemical Society (v.15-32,1862-1877).
4. Google Books is another excellent resource: http://books.google.com/
5. The NASA Technical Reports Server (NTRS) includes over 50,000 English translations of journal articles; conference papers; theses; technical reports,; ect., which appeared in print as NASA TT documents.
The availability of English translations of foreign language articles, changed with the closing of the Library of Congress' National Translation Center. This center contained the holdings of the SLA Translations Center, which was formerly located at the John Crerar Library. These translations were divided between the CISTI Library (1984+) and the British Library (pre-1984).