Detect language of text with web UI

[juhoinkinen]

Annif web UI could have a language detection function like is planned for Finto AI:

A possible enhancement for the Finto AI user interface would be to detect the language of input text. This would be easy to implement if the corresponding functionality is added to the Annif REST API: see #631

The /detect-language method was added to Annif REST API in #659, see also this Wiki page: https://github.com/NatLibFi/Annif/wiki/Language-detection

The functionality in the web UI should probably allow an user to define the candidate languages, in contrast to the Finto AI functionality where the candidate languages can be hard coded.

I create this issue is mostly to allow gathering feedback about the need for the feature. Our own production use probably won't benefit much of it.