Setting project to Wikimedia-Site-requests, as this request is about settings / configuration of a Wikimedia website.
There is nothing to change in the code base of the ULS software.

If you would like to write a software patch yourself, you will need to edit wmgULSWebfontsEnabled in https://phabricator.wikimedia.org/source/mediawiki-config/browse/master/wmf-config/InitialiseSettings.php and propose the patch in Gerrit.

I could, but unfortunately the login on Gerrit is broken. T220867

Change 509739 had a related patch set uploaded (by Vladis13; owner: Vladis13):
[operations/mediawiki-config@master] Enable webfonts for ru,uk,be of wiki,wikisource, and for sourceswiki

https://gerrit.wikimedia.org/r/509739

Enabling this in Wikisource seems ok, The webfonts feature when enabled by default in wikipedia, will cause large performance issues and this is the reason it is not enabled and left as a preference for user(See https://www.mediawiki.org/wiki/Universal_Language_Selector/WebFonts).

I don't think T156843: Adding fonts for Church Slavonic language in ULS is about wikipedia either. Correct me if I am wrong

How large performance issues can will? Will it a noticeable? From that link it not clear. AFAIU, the webfonts downloads once then caching for alltime.

In T220752#5266162, @Vladis13 wrote:

How large performance issues can will? Will it a noticeable? From that link it not clear. AFAIU, the webfonts downloads once then caching for alltime.

It has noticeable impact for users as well as WMF servers(last time we had to an emergency removal of feature due to perfromance hit). Also, this feature did not get any modernization for many years(webfont technology advanced in last few years). It is not a priority project for the language team as of now.

I removed Wikipedia from the commit, remains only Wikisources there.

Change 509739 merged by jenkins-bot:
[operations/mediawiki-config@master] Enable webfonts for ru,uk,be of wikisource, and for sourceswiki

https://gerrit.wikimedia.org/r/509739

Mentioned in SAL (#wikimedia-operations) [2019-08-28T11:15:11Z] <urbanecm@deploy1001> Synchronized wmf-config/InitialiseSettings.php: SWAT: 4ebddb8: Enable webfonts for ru,uk,be of wikisource, and for sourceswiki (T220752) (duration: 00m 55s)

@Vladis13: Hi! This task has been assigned to you a while ago. Could you maybe share an update? Do you still plan to work on this task, or do you need any help?

If this task has been resolved in the meantime: Please update the task status (via Add Action... → Change Status in the dropdown menu).
If this task is not resolved and only if you do not plan to work on this task anymore: Please consider removing yourself as assignee (via Add Action... → Assign / Claim in the dropdown menu): That would allow others to work on this (in theory), as others won't think that someone is already working on this. Thanks! :)

Enabled. Although seems that the extension don't work ... But there is no time for testing and no support, and formally this is another task.

In T220752#6068329, @Vladis13 wrote:

Enabled. Although seems that the extension don't work

@Vladis13: Thanks for the update! If something is broken, please follow https://www.mediawiki.org/wiki/How_to_report_a_bug and feel free to file a new task with clear and enough information, so others can try to reproduce. Thanks!