Learn English – Should foreign titles be capitalized according to English rules

Style guides may differ on this point—because it is strictly a matter of stylistic preference, not logical superiority—but most of the ones I checked don't address the question at all. The exception is R. M. Ritter, The Oxford Style Manual (2003), which discusses the issue in at least two places. First, in the context of main-text occurrences (at 4.1.8 "Titles and subtitles of works"):

Foreign-language titles and subtitles follow the rules common to that language. ... Except in general for French and German titles, this consists for the most part of minimal capitalization: capitalizing only the first word in the title and subtitle, and any proper names, Transliterated or romanized titles follow this practice as well.

Second, in the context of translations of titles to be cited in a bibliography (at 13.11.1.11 "Bibliographies," under the subhead "Styles of translation"):

Titles of works cited must follow English rules: italics for published books and titles of journals, roman quoted [that is, regular nonitalic text placed within quotation marks] for articles in journals and unpublished dissertations. ... Capitalization, however, follows the rules for the language of the title.

As applied to your situation, Oxford seems to recommend translating the title Lorem ipsum into its English equivalent—presumably something like Greeking text—but retaining the original's style of lowercasing the second word in the title. Thus:

Greeking text is a pretty interesting book.

If that doesn't seem to indicate with adequate clarity the fact that Greeking text is a book title, you can make that fact somewhat clearer by including the original-language version of the title in brackets (though Oxford does not endorse this practice):

Greeking text [Lorem ipsum] is a pretty interesting book.