Updated article: Styling using language attributes

The major change was the addition of detailed information about use of CSS selectors with xml:lang, but there were many other edits (see the list below). Translators should consider retranslating the whole tutorial. [search keys: qa-css-lang]

The article was updated as follows:

• added section “Using CSS selectors with xml:lang”
• the title was slightly changed
• information about browser support was replaced with a link to test results (updated)
• various edits throughout to improve readability
• removed the paragraph that says that generic class or id selectors work best, since support for selectors has significantly improved
• updated various links and added links to new materials.