While we strive to improve the default language selection in various places where we show only a subset of the available languages (like ULS, based on CLDR data), some users will always want to force a specific selection.

Several requirements/possible features have been identified for the manual selection:

1. apply to all devices;
2. apply to all wikis, i.e. be a cross-wiki selection;
3. be consistent for all products (ContentTranslation, WikibaseRepository etc. will need to adopt the same selection method);
4. ideally pre-fetch some reasonable per-user guesses, e.g. edit count in various wikis, interface language, fallback languages, home wiki;
5. ideally integrate with the feature showing the "most recent 2-3" languages used (e.g. by making this list transparent and adaptable).

Unlikely features:

1. select only the top10 wikis as https://github.com/he7d3r/mw-gadget-FilterInterlanguageLinks does;
2. guess languages from the content of local user contributions or currently accessed topic.

Possible storage methods:

1. user options in the CentralAuth wiki (sounds inefficient);
2. [[Talk:Universal_Language_Selector/Design/Interlanguage_links#Custom_CSS_rules|CSS rules]] in the global.css (feasible to edit in a way transparent to the local wiki user, e.g. in a ULS panel?);
3. {{#babel}} in a central wiki as Wikidata does (definitely too hack and an abuse of Babel).

Possible interfaces:

1. present a list of possible languages and let the user + them (T96547#1220434),
2. add a "blacklist" button next to each language guess,
3. add a "whitelist" or "edit list" button at the end of the list of language guesses