The stance that default languages don't exist is quite false in my opinion -- or a truism that it is not useful to solving the problem at hand. Almost every country has a language that is the dominant or national language, either officially or de facto. Or at least there can be some way of determining precedence for multiple languages.
It is often subjective or domain-specific what the primary language should be and what the weight a particular language should, and it is subject to change. So it makes sense that Java or Unicode does not provide this information. Luckily the number of countries with more than one language is quite small (at least by my JVM locale listing) and all the data we need to determine preference is easily available. So we can manually specify this information.
Below is my (subjective) ranking. Special care must be paid to some edge cases, e.g. Serbian, because Cyrillic is used in official communications but more people can read Latin. So it depends on your use case which one you prefer in some of these cases.
Also take care that the locale repository and language usage might change, so this answer may be outdated.
// https://en.wikipedia.org/wiki/Languages_of_Ireland
// English (99%)
// Irish (36%)
setPreference("IE", "en_IE", 0.99) // English
setPreference("IE", "ga_IE", 0.36) // Irish
// India
// https://en.wikipedia.org/wiki/Languages_of_India
// Hindi (57.1%)
// English (10.6%)
setPreference("IN", "hi_IN", 0.571) // Hindi
setPreference("IN", "en_IN", 0.106) // English
// Bosnia and Herzegovina
// https://en.wikipedia.org/wiki/Serbian_Cyrillic_alphabet
// Although the Bosnian language "officially accept[s] both alphabets", the Latin script is almost always used in the Federation of Bosnia and Herzegovina
setPreference("BA", "sr_BA_#Latn", 0.75) // Serbian
setPreference("BA", "sr_BA", 0.25) // Serbian
// Serbia
// https://en.wikipedia.org/wiki/Serbian_Cyrillic_alphabet
// Cyrillic is an important symbol of Serbian identity. In Serbia, official documents are printed in Cyrillic only even though, according to a 2014 survey, 47% of the Serbian population write in the Latin alphabet whereas 36% write in Cyrillic.
setPreference("RS", "sr_RS", 0.51) // Serbian
setPreference("RS", "sr_RS_#Latn", 0.5) // Serbian
// Belgium
// https://en.wikipedia.org/wiki/Languages_of_Belgium
// Dutch (1st language: ~55%, 2nd language: 16%)
// French (1st language: ~36%, 2nd language: ~49%)
setPreference("BE", "nl_BE", 0.55) // Dutch
setPreference("BE", "fr_BE", 0.36) // French
// Japan
setPreference("JP", "ja_JP", 0.51) // Japanese
setPreference("JP", "ja_JP_JP_#u-ca-japanese", 0.49) // Japanese
// Singapore
// https://en.wikipedia.org/wiki/Singapore#Languages
// Singapore has four official languages: English, Malay, Mandarin Chinese, and Tamil.[333] English is the common language, and is the language of business and government, and the medium of instruction in schools.
setPreference("SG", "en_SG", 0.369) // English
setPreference("SG", "zh_SG", 0.349) // Chinese
// Canada
// https://en.wikipedia.org/wiki/Languages_of_Canada
setPreference("CA", "fr_CA", 0.5597) // French
setPreference("CA", "en_CA", 0.2061) // English
// Switzerland
// https://en.wikipedia.org/wiki/Languages_of_Switzerland
// Year German French Italian Romansh Other
// 2015 63.0 22.7 8.4 0.6 5.3
setPreference("CH", "de_CH", 0.63) // German
setPreference("CH", "fr_CH", 0.227) // French
setPreference("CH", "it_CH", 0.084) // Italian
// Thailand
// https://docs.oracle.com/javase/tutorial/i18n/locale/extensions.html
// a Unicode locale extension is specified by the 'u' key code or the UNICODE_LOCALE_EXTENSION constant. The value itself is also specified by a key/type pair. Legal values are defined in the Key/Type Definitions table on the Unicode website. A key code is specified by two alphabetic characters.
// nu number type
setPreference("TH", "th_TH", 0.51) // Thai
setPreference("TH", "th_TH_TH_#u-nu-thai", 0.49) // Thai
// Luxembourg
// https://en.wikipedia.org/wiki/Languages_of_Luxembourg
// 2012 Luxembourgish French German English other
// Native language 52% 16% 2% N/A 30%
setPreference("LU", "fr_LU", 0.16) // French
setPreference("LU", "de_LU", 0.02) // German
// Montenegro
// https://en.wikipedia.org/wiki/Montenegrin_alphabet
// Although the Latin and Cyrillic alphabets enjoy equal status under the Constitution of Montenegro, the government and proponents of the Montenegrin language prefer to use the Latin script.
setPreference("ME", "sr_ME_#Latn", 0.8) // Serbian
setPreference("ME", "sr_ME", 0.2) // Serbian
// United States
setPreference("US", "en_US", 0.98) // English
setPreference("US", "es_US", 0.88) // Spanish
// Malta
// https://en.wikipedia.org/wiki/Languages_of_Malta
// 98% of Maltese people can speak Maltese, 88% can speak English
setPreference("MT", "mt_MT", 0.98) // Maltese
setPreference("MT", "en_MT", 0.88) // English
// Spain
// https://en.wikipedia.org/wiki/Languages_of_Spain
// the most prominent of the languages of Spain is Spanish (Castilian), spoken by about 99% of Spaniards as a first or second language.[5] Catalan (or Valencian) is spoken by 19%
setPreference("ES", "es_ES", 0.99) // Spanish
setPreference("ES", "ca_ES", 0.19) // Catalan
// Norway
// https://en.wikipedia.org/wiki/Languages_of_Switzerland
// Bokmål is the preferred written standard of Norwegian for 85% to 90%
// Nynorsk is reportedly used as main form of Norwegian by around 7.4% of the total population
setPreference("NO", "no_NO", 0.85) // Norwegian
setPreference("NO", "no_NO_NY", 0.074) // Norwegian
// Greece
// The official language of Greece is Greek, spoken by 99% of the population
// English (51%)
// German (9%)
// French (8.5%)
// Italian (8%)
setPreference("GR", "el_GR", 0.99) // Greek
setPreference("GR", "de_GR", 0.09) // German
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