Add validation for messageformat interpolation

[garethbowen]

If a translation value changes the variable name then the translation doesn't work. Add a validation check to ensure that the variable name in each translation file matches the reference (English) translation.

Steps to reproduce:

1. Have an English file with line: test = {DAYS, plural, one{1 day left} other{\# days left}}
2. Have a French file with line: test = {JOURS, plural, one{1 jour restant} other{\# jours restants}}

Expected: Validation failure
Actual: No warnings

If this is missed you end up with issues like: medic/cht-core#9384

[garethbowen]

@1yuv Is this something you'd be willing to pick up?

[1yuv]

Hi @garethbowen , If I read the document correctly, I think this is already supported

[garethbowen]

@1yuv In that example, I think it checks the "uno" but not the "MONTHS". This would be an expansion of that. I'm confident it's not already working because if it was then the French issue you found would have been picked up in the linting stage.

[1yuv]

I see that, I thought we were being lenient with use of this tool. I unfortunately don't have a bandwidth to take this immediately. I will pick this in October if it's not worked on.