Feature summary (what you would like to be able to do and where):

In Wikipedia, the language dropdown shows lots of different languages with a large set of "Suggested languages at the top and all the rest sorted by continent.

The place of the links for the languages actually relevant to the user always vary and have first to be manually found by manually looking through the of the long list of languages.

Most users aren't very interested in dozens or hundreds of other language versions because they only speak one to four languages.

Enabling the user to pin a few languages they'd like to see in the always same order always at the top would make it much easier and quicker to switch between languages.

The image shows how it could look like and better illustrates what I mean:

Here the user has pinned the following languages: German, French, Spanish, Simple English, English (article open is English)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

For example, I'd like to set English, German, simple English Wikipedia and maybe a few more that I can manage to understand and write somewhat. Just with a click I could then see whether/which of these exist and consider switching to them. THis is also useful in smaller language versions where if English is set as preferred language there, it becomes much easier to switch to the ENWP article which very often is much more comprehensive and up-to-date.

Benefits (why should this be implemented?):

Enabling the user to pin a few languages they'd like to see in the always same order always at the top would make it much easier and quicker to switch between languages.

Additionally, it makes it much much easier for users to see that a simple English Wikipedia version exists and quickly switch to it in case the article is too long and complex (one of the biggest Wikipedia problems in the real world today). It also motivates users to take a short look at that dropdown more often as they will be able to expect to find out whether it exists in their preferred alternative language(s) fairly quickly and without mental exhaustion.

One could even use blurred colors in the background of the dropdown with one color assigned to each of 10 maximum preferred languages and then color the large button accordingly so one can see without even clicking it in which of the languages it's available in. T344028: The "missing relevant languages banner" with slow loading makes the interwiki links jump down addressed parts of the problems to some extent; having pinned language would address it much further. Note that instead of uselessly showing random languages the article is not available in, that panel would then only show the pinned languages the article is not available in so as to be of actual relevance to the user. The pencil edit button allows to easily reconfigure the pinned languages in a self-explanatory way (this can also be done via the settings wheel in the bottom right).

This also incentivizes users to register on Wikipedia and log in / stay logged in because they then can configure their preferred languages and more easily switch to them.

Adding functionality for pinned languages helps the information needs of the readers and makes them stay within the Wikiverse instead of going off-site to look for the info elsewhere (e.g. to a website that explains it in simpler terms or a site that has the information that the DEWP article does not but which would be in the ENWP article) or to switch to their language version whether some info in the article is also present there to improve it if needed etc. There's many reasons for why improving language switching has lots of benefits.