How to change case of interpolated variables in Rails locale file?

Question

Using Rails 3.1.3 with Ruby 1.9.3p0.

I\'ve discovered that by default, Rails does not use sentence case for form buttons. For example, instead of an \"Update user\" but

Answer 1

After further research, I've concluded that this kind of operation on interpolated values is not possible, at least using a YAML locale file.

YAML is documented here and doesn't support string operations:
http://www.yaml.org/spec/1.2/spec.html

The main page on Ruby localization is here:
http://ruby-i18n.org/wiki

From there, we find the code for the default I18n gem and drill down to the interpolation code. It uses sprintf to do the interpolation:
https://github.com/svenfuchs/i18n/blob/master/lib/i18n/interpolate/ruby.rb

That code is "heavily based on Masao Mutoh's gettext String interpolation extension":
http://github.com/mutoh/gettext/blob/f6566738b981fe0952548c421042ad1e0cdfb31e/lib/gettext/core_ext/string.rb

That extension has an example of formatting numbers:

For strings.
"%{firstname}, %{familyname}" % {:firstname => "Masao", :familyname => "Mutoh"}

With field type to specify format such as d(decimal), f(float),...
"%d, %.1f" % {:age => 10, :weight => 43.4}

The extension refers to the [Ruby] "Kernel::sprintf for details of the format string":
http://www.ruby-doc.org/core-1.9.2/Kernel.html#method-i-sprintf

In that doc on sprintf, there are lots of ways to format numbers, but no operations for changing the case of strings.