[QA] Language tags should be chosen from a list

# Current situation

When configuring a web service in GERBIL QA, the user has to define the language tag which is supported by its web service. This has two disadvantages:
1. A user may mistype the tag
2. A user might not be aware of the correct tag or might choose a tag that won't work with the chosen dataset(s)

# Solution

- [ ] Define the languages that a dataset supports in the dataset's metadata
- [ ] Offer those languages in a list that are supported by all chosen datasets

Note: this may not work when a user has uploaded a dataset. This could simply cause a hint in the UI.

Start by tracing how GERBIL QA reads dataset metadata and builds the web-service configuration UI. Define supported languages in dataset metadata, offer the intersection for selected datasets, and show a hint when an uploaded dataset cannot provide this information.

Written by the indexing model from the issue text.