Fix untranslatable string in advancefrom deprecated message

[fujimo-t]

I noticed that second chunk is not marked to translatable while working 1.16 translation.

The target branch is set to master because I heard 1.16 is string freezed.
Is it possible to merge this also 1.16 and re-run pot update?

[stevecotton]

I think this one shouldn't be translatable at all, as it's an error that will only make sense to WML authors. @ivanovic?

[CelticMinstrel]

Our generic "X has been deprecated" message is translatable, so I think it can make sense to make "use Y instead" translatable too… but probably it should be a single string, like "use $elem instead" and substitute in the replacement?

That doesn't quite work here, mind you, but other deprecation messages have an untranslatable portion with additional details, so perhaps the same could be done here.

interpolate_into_string(
  _("Use $elem instead."),
  {"elem", "[modify_unit_type]\n" + modify_unit_type.debug() + "\n [/modify_unit_type]"
) + "This goes in the [campaign] or [multiplayer] tag rather than in the [unit_type]."

However, in that case it's probably something that has to wait for 1.16.1?

[fujimo-t]

Indeed, using substitution looks to better and this untranslatable thing is not important for most players.

[Wedge009]

This seems to be hanging around unnecessarily, given it's a small change. If waiting for 1.16.1, it can just be added to the back-port list, no?

Does fujimo need help adapting celmin's suggestion?

[fujimo-t]

I'm sorry for late reply.
I had left GitHub for a while.

I will try to fix by string interpolation.

[stevecotton]

This message never triggers, the code to trigger it is broken and looks for [units][advancefrom] instead of [unit_type][advancefrom]. This is issue #6204.

If we're going to make it a translatable player-readable string, I suggest phrasing it as Error: [advancefrom] does not work, "Cuttle Fish" units will not be able to advance to "Kraken". Even if the unit names aren't translated, that's probably the info the player needs to decide whether to wait for the campaign author to fix it.

[stevecotton]

Many thanks to you @fujimo-t, but as I've already written other C++ changes from investigating #6204 it felt logical to open a new PR that supersedes this one.