WP15 Theme: Allow translators to disable Google Fonts for their language

[dimadin]

Attached is a patch that takes approach from default WordPress themes. Note that subset query arg ​takes effect only for browsers that don't support unicode-range.

[iandunn]

@melchoyce, do you have any preferred backup fonts for locales where Crete Round and/or Source Sans Pro are disabled?

[melchoyce]

For the sans serif, we'll probably want to look at Twenty Seventeen's font stack. You can't really go wrong with Arial generally.

Crete Round is more of a stylistic choice, so in that case, it really depends on the language/alphabet. I don't want to say "go with a serif," because some alphabets don't necessarily have serifs — and if they do, they don't always look great. (I feel like I remember someone — maybe @ranyanivhartstein? — saying that Hebrew serif fonts were pretty illegible?)

Would love to get some suggestions if we have designers with experience in non-latin alphabets.

[SergeyBiryukov]

WordPress core and bundled themes generally use Tahoma for RTL, except for Hebrew, which uses Arial. Twenty Seventeen seems to handle this correctly.

3569.diff​ looks good to me.

[iandunn]

In 7117:

WP15: Only enqueue fonts for locales that they support.

Props dimadin.
Fixes #3569.