A few notes:

Wikis must use the icu collation library in order to have numeric collation. Some are already using it. English didn't want to, need to find out why.

For wikis that are using the icu library, you just have to change the configuration for it (in Collation.php). See T8948#1919137.

The existing config variable for turning on icu collation is $wgCategoryCollation.

For non-icu wikis, it converts the page title to uppercase and then stores as binary UTF-8. The sorting is based on that.

There's a lot of notes from Bartosz and Bawolff on the top 10 notes page:
https://meta.wikimedia.org/wiki/2015_Community_Wishlist_Survey/Top_10#Numerical_sorting

Related issues discussed in these 2 tracking tasks:

• T32996: Change $wgCategoryCollation values to appropriate one for each Wikimedia wiki
• T32673: Implement central locale-specific, or tailored, sorting framework (tracking)

In T120854#1928338, @DannyH wrote:

There's a lot of notes from Bartosz and Bawolff on the top 10 notes page:

now at https://meta.wikimedia.org/wiki/Community_Tech/Numerical_sorting_in_categories

Here's a short preliminary investigation:

Most of what we need here is covered by Bartosz's summary here: https://meta.wikimedia.org/wiki/Community_Tech/Numerical_sorting_in_categories/Notes#Message_from_Bartosz.2C_Dec_22

ICU library does support numerical sorting as can be tested at https://ssl.icu-project.org/icu-bin/collation.html (Turn numeric sorting on, give different inputs and click Sort)

Bawolff's comment here: T8948#1919137 describes how this can possibly be implemented.

We'd probably need to tweak the implementation to work with different languages which might be a bigger problem. Also keep RTL languages as a factor in mind.

A note from @matmarex on the Meta talk page, about supporting the umlauts:

"This should be entirely possible today (without any further development work), by setting $wgCategoryCollation to uca-de. The only problem could be the updateCollation script taking too long, but dewiki categorylinks table has only ~10M rows too."

Here are the next steps:

• Improve performance of updateCollation script (T58041) - This is still moving along. There is a patch (https://gerrit.wikimedia.org/r/#/c/272416/), but it requires further testing and investigation, which @jcrespo is working on. There's not a lot we can do on this particular part to help other than shepherding the patch.
• Figure out if it’s necessary to overload $wgCategoryCollation with an option for numerical sorting. It's possible that all WMF wikis, even Wiktionary, will want to use natural sorting, in which case, we may be able to do something relatively simple, rather than creating a whole new set of collation options. See T8948#2076236.
• Fix the numerical header bug (T128483).
• Test the ICU numerical sorting feature on test wiki.
• Test the ICU numerical sorting feature on a real wiki.
• Change the default setting for $wgCategoryCollation to use numerical sorting.

Wikimedia wikis that want this can now request it. To do so:

1. Please start a community discussion – RfC, vote, or however your wiki normally decides these things – to make sure there’s support for it.
2. Once you’re sure it has support, post on User:DannyH (WMF)’s talk page on Meta to with a link to the discussion where you took the decision.

(Translatable instructions.)