i18n plugin missing basic language versions for languages where country-specific versions exist

There are a number of languages in the i18n plugin where there are country-specific versions of the language files but no basic versions. For example there are sv-SE.* files but no sv.* files.

This causes problems for applications that choose a language file, eg using PHP's Locale::lookup() method, based on a user-specified locale, either via a config setting or via the browser language preferences. For example, if the locale is set to 'sv' then the 'sv-SE' file will not be found. The RFC 4647 lookup algorithm used by Locale:lookup() allows for falling back from, eg, 'sv-SE' to 'sv' but not the other way round.

There should be a basic language file for all languages where there is a country-specific version. In most cases this will be a simple matter of copying the main country-specific version, eg 'sv-SE' to 'sv, 'de-DE' to 'de', 'nl-NL' to 'nl', etc.

Inspect the i18n plugin's locale files, starting with the sv-SE.*, de-DE, and nl-NL examples mentioned in the issue. Identify every country-specific locale without a basic counterpart, add the corresponding basic language files, and verify that each affected language now has one.