Improve error messages in case of missing translations

[wwalc]

🐞 Is this a bug report or feature request? (choose one)

• Bug report 😈

💻 Version of CKEditor

1.0.0-alpha.2

📋 Steps to reproduce

1. Create documentation for CKEditor 5 with Umberto.

✅ Expected result

Better information about where precisely the translation is missing and for which language. I may be interested only in certain languages and be fine with missing translations in other languages.

❎ Actual result

📃 Other details that might be useful

[ma2ciek]

That should be fixed by the #624. The language will be added to the error message.

[ma2ciek]

Hmm, after changes in the #624, there will be a lot of errors for the default build (additionalLanguages: all). Do we want to collect those errors and display something like this: "Not found 13 translations for 'de' language."? On the other hand, that label won't be very helpful.

cc @Reinmar @szymonkups

[Reinmar]

But those warnings should only be logged if you chose a specific language. If you build for 'all' they make little sense because you're not interested in 99% of those languages. In fact... perhaps these warnings make no sense at all?

[ma2ciek]

In fact... perhaps these warnings make no sense at all?

I was thinking about this, but the fact, that some translations are missing should be important for the developer, at least, when he selects one target language. But I know, he can't do anything at this stage... (despite adding the missing translation on Transifex :D)

These errors (or stats) makes perfect sense after downloading translations from Transifex.

[Reinmar]

Let's do this:

• by default 0 warnings should be logged as they don't make sense most of the time,
• but you should be able to turn on a verbose mode in the plugin and in such a case it should log everything.