Fix Interslavic BCP47 code

[noomorph]

Hi, these locales look like workarounds, can we align them to the actual BCP 47 standard, please?

Invalid locales cause issues like this:

Intl.ListFormat.supportedLocalesOf(['isv_l', 'en']);
VM221:1 Uncaught RangeError: Invalid language tag: isv_l
    at ListFormat.supportedLocalesOf (<anonymous>)
    at <anonymous>:1:17

Even linku.la crashes if I choose those invalid locales, e.g., on this page: https://linku.la/words/kalama?q=

[noomorph]

If this is a problem on your CrowdIn, please fix those codes there and feel free to close my PR. 🙏

[noomorph]

@gregdan3 sorry for the ping, may I know some contacts if this is out of your domain? You can find me on Toki Pona's Discord if needed.