Add easy way to swicth the primary NotoSansCJK font

[Huulivoide]

Currently Japanese variant of NotoSansCJK is used by default, unless another font is requested specifically. For me this is a problem as I need the Chinese variant by default as many applications don't allow you to change the font per language.

[bohoomil]

This is actually one of the problems I've never managed to solve properly... There is no obvious way to make fontconfig use a particular font for a given CJK script: although it's possible, the results happen to be unreliable and inconsistent, especially when we have more than one family providing support for a certain language. On the other hand, Firefox renders Japanese and Chinese scripts with dedicated fonts, though: for zh.wikipedia.org it picks Noto Sans CJK SC, while for jp.wikipedia.org the JP equivalent.

Which applications in particular do you find most affected by the issue?

[Huulivoide]

The biggest problem comes down to being ibus and to some extend Firefox. With ibus the problem is bigger as I cannot sometimes find the correct character form the list of offered ones. With Firefox the problem is still there, sadly most Chinese websites (that I use) do not specify the HTML lang attribute anywhere and as so Firefox just falls back to general font, instead of the one I have specified for Chinese in the settings.

I think the easiest, but not cleanest one, would simply to be to split the CJK font's off the noto-sans-multilingual package and allow user to omit from installing all of them, thus allowing one to choose one that fits his/her needs best.

[LQYMGT]

As this page said, we can have different default languages.

It is recommended to use "Noto Sans CJK {JP, KR, SC, TC}" rather than "Noto Sans {JP, KR, SC, TC}", unless space/latency is very restricted. Each of the font families "Noto Sans CJK {JP, KR, SC, TC}" supports all four languages, but has a different default language.

Maybe this can help?