The current ULS uses the following logic

/**
 * Get a list of codes for languages to show in
 * the "Common languages" section of the ULS.
 * The list consists of the user's current selected language,
 * the wiki's content language, the browser' UI language
 * and Accept-Language, user's previous selected languages
 * and finally, the languages of countryCode taken from the CLDR,
 * taken by default from the user's geolocation.
 *
 * @param {string} [countryCode] Uppercase country code.
 * @return {Array} List of language codes without duplicates.
 */
mw.uls.getFrequentLanguageList = function ( countryCode ) {
	const ret = [],
		lists = [
			[
				mw.config.get( 'wgUserLanguage' ),
				mw.config.get( 'wgContentLanguage' ),
				mw.uls.getBrowserLanguage()
			],
			mw.uls.getPreviousLanguages(),
			mw.uls.getAcceptLanguageList()
		];

	countryCode = countryCode || mw.uls.getCountryCode();
	if ( countryCode ) {
		lists.push( $.uls.data.getLanguagesInTerritory( countryCode ) );
	}
        ....
}

Change #1239999 had a related patch set uploaded (by Abijeet Patro; author: Abijeet Patro):

[mediawiki/extensions/UniversalLanguageSelector@master] ULS rewrite: Display supported languages

https://gerrit.wikimedia.org/r/1239999

In T287877#11616321, @abi_ wrote:

The current ULS uses the following logic

/**
 * Get a list of codes for languages to show in
 * the "Common languages" section of the ULS.
 * The list consists of the user's current selected language,
 * the wiki's content language, the browser' UI language
 * and Accept-Language, user's previous selected languages
 * and finally, the languages of countryCode taken from the CLDR,
 * taken by default from the user's geolocation.
 *
 * @param {string} [countryCode] Uppercase country code.
 * @return {Array} List of language codes without duplicates.
 */
mw.uls.getFrequentLanguageList = function ( countryCode ) {
	const ret = [],
		lists = [
			[
				mw.config.get( 'wgUserLanguage' ),
				mw.config.get( 'wgContentLanguage' ),
				mw.uls.getBrowserLanguage()
			],
			mw.uls.getPreviousLanguages(),
			mw.uls.getAcceptLanguageList()
		];

	countryCode = countryCode || mw.uls.getCountryCode();
	if ( countryCode ) {
		lists.push( $.uls.data.getLanguagesInTerritory( countryCode ) );
	}
        ....
}

The current implementation includes everything mentioned above minus the languages in user territory. We can add that once we've included language data library.

Change #1239999 merged by jenkins-bot:

[mediawiki/extensions/UniversalLanguageSelector@master] ULS rewrite: Display suggested languages

https://gerrit.wikimedia.org/r/1239999

Here's how suggested languages are currently displayed:

1. previously selected languages
2. user language (wgUserLanguage)
3. current wiki content language (wgContentLanguage)
4. browser language
5. accept language (wgULSAcceptLanguageList)

All items from the task are done. I'll create a separate task to also add languages based on the region into the list of suggestions