Allow selecting label language in suggest operations

[osma]

The suggest operation (both CLI and REST) always returns the subject labels in the configured vocabulary language, even though the vocabulary could include labels in other languages. For example, YSO is trilingual (fi, sv, en) and thus the subject labels could be given in any of these languages.

We could make it possible to select the subject label language in suggest operations. In the CLI, it could be a parameter -L / --language:

annif suggest -L en yso-mllm-fi <document.txt          # return English language subject labels
annif suggest --language en yso-mllm-fi <document.txt  # same but with long option

In the REST API, it could be a parameter lang or language that works similarly to the current limit and threshold parameters.

Currently the Finto AI user interface has similar functionality, but it relies on the Finto REST API to fetch labels in non-default languages. Having this available in the Annif API would simplify the implementation.