Try langid or hyphenation instead of (or in addition to) language.
See https://tex.stackexchange.com/questions/96469/switching-languages-in-bibliography-with-biblatex

+++ Robert Goulding [Aug 06 17 14:52 ]:
>   Escaping the colon works perfectly, after testing.
>   I had one other query.  When I convert to docx, the foreign titles are
>   put into sentence case, despite the fact that I have the language field
>   set.  So, for example, the entry in my bibtex file:
>   @BOOK{Weinrich1998-zr,
>     title     = "Die Lichtbrechung in den Theorien von Descartes und
>   Fermat",
>     author    = "Weinrich, Klaus",
>     publisher = "F. Steiner",
>     series    = "Sudhoffs Archiv: Beihefte",
>     year      =  1998,
>     address   = "Stuttgart",
>     language  = "de"
>   }
>   turns up in my bibliography as: Weinrich, Klaus. 1998. Die
>   Lichtbrechung in Den Theorien von Descartes Und Fermat. Sudhoffs
>   Archiv: Beihefte. Stuttgart: F. Steiner.
>
>   with "den" and "und" capitalized (but not "von"!).  I see this behavior
>   whether or notI explicitly include the Chicago csl file from the Zotero
>   repository, or just use the pandoc default.
>   Robert.
>   On Sunday, August 6, 2017 at 5:33:36 PM UTC-4, Robert Goulding wrote:
>
>   Thank you! I will backslash the colons in the file as a fix for now.  -
>   Robert.
>   On Sunday, August 6, 2017 at 12:02:43 PM UTC-4, John MacFarlane wrote:
>
>     Thanks (and apologies if I missed the files before).
>     Simple repro here:
>     ```
>     hi[^fn3]
>     [^fn3]: [@Alhazen1572-qk, V.9]: "competentius est et rationabilius,
>     ut appareat supra perpendiculum, quam extra eam."
>     ```
>     This yields an empty footnote.  Why?  Because the footnote
>     contains:
>     ```
>     [@Alhazen1572-qk, V.9]: "competentius est et rationabilius, ut
>     appareat supra perpendiculum, quam extra eam."
>     ```
>     which pandoc is parsing as a reference link definition!
>     Perhaps pandoc should not allow reference link definitions
>     that start with `@`, since these conflict with citations?
>     Anyone have thoughts about this?  Someone should probably
>     put an issue on the bug tracker for this.
>     In the mean time, a workaround is to backslash-escape the
>     colon:
>         [@Alhazen1572-qk, V.9]\: "competentius...
>
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