Core: Translate directive and pipe not perform the same way

[justforuse]

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[ ] feature request

Current behavior
translate directive and pipe dont transform unknown value in same way
when directive meet unknown value output the value itself, but cannot changed.
pipe performs well

Reproduction of the problem

https://stackblitz.com/edit/angular-qjsz5l

• ngx-translate version: 9.1

• Angular version: 5.2

[ar53n]

And pipe transform show first empty string but directive show first path to translate string.

[91K00]

I detect also some weirds behaviours when you add content inside the element that is translated.
These two snippets are NOT working properly:

<el1 [translate]="'I18N.KEY'"> <el2>other content</el2> </el1>
Displays only the "other content" skipping the translation.

<el1 translate>I18N.KEY <el2>other content</el2> </el1>
The translation is not anymore translated on language change.

Here is the stackblitz : https://stackblitz.com/edit/91k00-ngx-translate-bug