Check valid positional substitution in internationalization strings

[eharris]

The simplest version of this might just check for matched braces for positional substitutions ({0}) in all the internationalization files, so for any { make sure it is followed by integer followed by close brace, and for any } verify it is proceeded by open brace plus integer.

An even better check would be that the number of used positional substitutions matches the number provided when they are used, but that is probably too complicated to be worth it.

This was recently discovered as a long standing problem in airsonic/airsonic#1298