of Unicode and non-ascii string that are incompatible (because the
encoding of the latter is unknown) by substituting a representation of
the non-ascii string.

I18n interpolation doesn't fail anymore if a i18n:name is not provided,
the ${string} in the translation is just left as is.

Collector #696: tal:replace of a non-string (a number for examlpe)
associated with a i18n:name failed to be interpolated properly.

Improved tests for i18n and added tests for interpolate().