I think the best approach is to create some macro-magic in two similar pipeline stages that some command-line option inserts in the compiling pipeline...

Personal preference: I would prefer to have "internationalized string literals" denoted by a # prefix.

When a string _"test" is marked the compiler would replace the literal by a call to an ITranslatableStringHandler.GetString("test") method.

Boo.Lang.I18N.Globalization.SourceCulture as CultureInfo : culture of the marked strings (can be null for undefined).
Boo.Lang.I18N.Globalization.TranslationCulture as CultureInfo : if null will take the culture of the CLR else force translate to TranslationCulture if available.
Boo.Lang.I18N.Globalization.TranslatableStringHandler as ITranslatableStringHandler : the default could be a ResourceManager-based implementation, user can set this static property for using its own implementation (using Mono.GetText for instance).

Boo.Lang.I18N.ITranslatableStringHandler
GetString(original as string) as string
FormatString(original as (string), arguments as (object)) as string

Marked string interpolations would be handled by FormatString() with the string split between the arguments.

Just my 2 cents,

Are you willing to contribute?