docs: Clarify config for custom interpolation suffix/prefix

[joostdecock]

It took me a while to figure out how to get a custom interpolation prefix/suffix to work.

I think it's a little unintuitive that changing an option that relates to how the text inside the translation files is structured has a knock-on effect to how files are loaded. That is not criticism, I realize it's because of how the various packages interact, but it's still a bit of a head-scratcher.

After doing some digging, I came across #1090 that clarifies the issue, but I feel it's worth putting this in the README.

This PR does that. It's not much, but I hope you find it useful.

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[isaachinman]

Hey @joostdecock – #1248 was supposed to fix this issue, and localeStructure should respect the interpolation options. Are you saying you've noticed a regression in that regard?

[joostdecock]

It works, but only if you (also) add the localeStructure setting.
If my reading of #1248 is correct, that is also what is supposed to happen (as in, the example clearly sets the localeStructure setting.

This example lifted from #1248 will work as-is:

module.exports = {
  i18n: {
    defaultLocale: 'en',
    locales: ['en', 'de'],
  },
  interpolation: {
    prefix: '%{',
    suffix: '}',
  },
  localeStructure: '%{lng}/%{ns}',
}

But removing the localeStructure setting will make this not work:

module.exports = {
  i18n: {
    defaultLocale: 'en',
    locales: ['en', 'de'],
  },
  interpolation: {
    prefix: '%{',
    suffix: '}',
  }
}

So it's easy enough to get it to work if you know that you must set localeStructure to match your custom prefix/suffix.
I found that confusing at first since the prefix/suffix setting relates to how placeholders for interpolation are escaped inside your data. So it's not really obvious that this has a knock-on effect on locating the files themselves.

So I felt some clarification would be best.
I hope I'm managing to explain myself 😂

[isaachinman]

Yep, makes sense, and thanks for a detailed explanation. In the end, these are just raw, user-config strings, and it wouldn't be possible to detect and swap custom prefix/suffix values.

Happy to merge this, thanks very much for your contribution!