Fix (i18n): Return the Correct Locale from Filenames - fixes #5909

[andipabst]

Summary
When determining the locale from the filename in a i18n.structure: multiple_files-setup, the code returned g for the file index.gd.md. This was due to the use of the lodash-method trimEnd for removing the file extension.

As per the lodash docs the trimEnd method does not remove the specified string from the end, but instead removes all characters from the end. So in this case when calling

trimEnd('index.gd.md', '.md')

it did not just remove the '.md' at the end, as intended, but instead all occurrences of the characters ., m and d from the end. Thus the result was index.g instead of index.gd.

I changed the implementation to use the String.prototype.slice() method to remove the number of characters of file extension from the end of the string.

This closes #5909.

Test plan
I added jest tests.

Checklist

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[erezrokah]

@andipabst, this is 🚀

And also 🤦 since I'm the one who missed on this