Someone using a non-English UI may wish to modify/add to this post as appropriate.
1. One can limit the search to a particular language by using a search argument in the form of "language colon word". Note that the behavior of "match all forms" will vary.

2. When the language is transliterated, Logos does not tag with the language but with transliterated. This is abbreviated to translit in the search argument.

3. One can get more complete results for a search by searching both for the original language and the transliteration.

4. If your library is multilingual (mine is not a good example), you can broaden searches to include multiple language resources in a single search. Note the increase in hits.

What languages can be searched?
... for all languages "X" that Logos supports (which is basically any that have an ISO-639 language code).
How accurate is the coding?
All text in a Logos resource is tagged with its language, so all resources are coded for (foreign) languages.
To clarify: "[In theory, a]ll text in a Logos resource is tagged with its language, so all resources are [supposed to be] coded for (foreign) languages."
Foreign language strings will generally not be tagged unless they're set in italic typeface in the book, which is a (weak) indication that the author considered the word or phrase to be foreign. So, for example, if an author italicizes "Weltanschauung" it will probably be tagged as German; if the author casually uses it in plain text without calling it out, then maybe/probably not. These days, words like "zeitgeist" and "schadenfreude" show up in my English spell-checker by default, but books of older eras called them out as foreign.
Book titles and abbreviations are an especially gray area case, and I suspect the editorial policies over the many years have been neither 100% consistent nor applied 100% consistently. We've also corresponded before about whether or not a Latin phrase in common usage (such as "e.g." or "per se") or technical terminology (such as biological nomenclature) has crossed the language barrier.