It seems like strings localized with the [ https://commons.wikimedia.org/wiki/Template:No | no template ] does not work as expected when data is requested by the API query for image props, like [ https://no.wikipedia.org/w/api.php?action=query&format=json&prop=imageinfo&titles=File:Hovedbygget%20ntnu.jpg&iiprop=timestamp|user|url|size|mime|mediatype|extmetadata&iiextmetadatafilter=DateTime|DateTimeOriginal|ObjectName|ImageDescription|License|LicenseShortName|UsageTerms|LicenseUrl|Credit|Artist|AuthorCount|GPSLatitude|GPSLongitude|Permission|Attribution|AttributionRequired|NonFree|Restrictions&iiextmetadatalanguage=no | this call ]. Note that the "ImageDescription" is in English, yet there is a provided description in Norwegian. This image can be found at [[ https://commons.wikimedia.org/wiki/File:Hovedbygget_ntnu.jpg | File:Hovedbygget ntnu.jpg ]].
A similar problem existed at [[ https://commons.wikimedia.org/wiki/File:Nasjonalparker_Syd-Norge.JPG | File:Nasjonalparker Syd-Norge.JPG ]], and by changing it to use the [ https://commons.wikimedia.org/wiki/Template:Nb | nb template ] it starts working, like [[ https://no.wikipedia.org/w/api.php?action=query&format=json&prop=imageinfo&titles=File:Nasjonalparker%20Syd-Norge.JPG&iiprop=timestamp|user|url|size|mime|mediatype|extmetadata&iiextmetadatafilter=DateTime|DateTimeOriginal|ObjectName|ImageDescription|License|LicenseShortName|UsageTerms|LicenseUrl|Credit|Artist|AuthorCount|GPSLatitude|GPSLongitude|Permission|Attribution|AttributionRequired|NonFree|Restrictions&iiextmetadatalanguage=no | this call]] shows.
The URLs came originally from requests made by MediaViewer and is now used in a similar form in some gadget at nowiki. The same errors are clearly shown at nowiki, for example at [[ https://no.wikipedia.org/wiki/Folgefonna_nasjonalpark#/media/File:Nasjonalparker_Syd-Norge.JPG | Folgefonna nasjonalpark ]] (with MV open, correct description) and at .. err, I used a previous version of Folgefonna nasjonalpark for checking this. The article at NTNU has a local text at nowiki. I try to find another example later on.
It seems like both "no" and "nb" will get image descriptions marked with the "nb" template, but neither work against the "no" template.
A fix could be to simply run a bot and change all the "no" templates to "nb" templates, or perhaps even just make "no" a redirect to "nb". Before I do anything like that I would give you guys some time to figure out if this is a bug or a feature.