Language tags are case-insensitive. Closes #517 - #528

It does not matter what Chrome or Firefox thinks; it matters what BCP 47 thinks; BCP 47 says case insensitive. I speak "en-US", and you want SVG 2 to say that language is different from "en-us"? BCP 47 says the preferred capitalization is "en-US", but comparisons do not matter. But Chrome and Firefox recognize that the comparison should be case-insensitive. Chrome/Blink is working to make systemLanguage case-insensitive. See https://bugs.chromium.org/p/chromium/issues/detail?id=869440 and the fsoder comment on issue #517. Firefox's nightlies are now case-insensitve. See https://bugzilla.mozilla.org/show_bug.cgi?id=1480946 and the longsonr comment on issue #517. And CSS says the :lang psuedo selector does case insensitive comparisons. W3C sez The pseudo-class ':lang(C)' matches if the element is in language C. Whether there is a match is based solely on the identifier C being either equal to, or a hyphen-separated substring of, the element's language value, in the same way as if performed by the '|=' <https://www.w3.org/TR/CSS21/selector.html%23id-selectors#attribute-selectors> operator. *The matching of C against the element's language value is performed case-insensitively for characters within the ASCII range.* The identifier C does not have to be a valid language name. https://www.w3.org/TR/CSS21/selector.html%23id-selectors#lang The pseudo-class :lang(C) represents an element that is in language C. Whether an element is represented by a :lang() selector is based solely on the element's language value (normalized to BCP 47 syntax if necessary) being equal to the identifier C, or beginning with the identifier C immediately followed by "-" (U+002D). *The matching of C against the element's language value is performed case-insensitively within the ASCII range.* The identifier C does not have to be a valid language name. https://drafts.csswg.org/selectors-3/#lang-pseudo (some emphasis added)