Transloco: Allow fallback language for missing values

[NetanelBasal]

Example: If I use Spanish, and one of the keys is missing, I want to display the English version of it.

[edbzn]

@NetanelBasal I would be happy to pick this.

[NetanelBasal]

Go ahead 😀

[edbzn]

@NetanelBasal In your point of view is it better to add this behavior directly in the translate method or delegate this to the DefaultHandler ?

[NetanelBasal]

There are couple of issues with this approach:

1. It will not work with the structural directive as we don't support it with the missing handler.
2. When we have missing value, we need to load the fallback language, something that could lead to a bad UX such as jumpy content.

The solution for this is that if the consumer passes the fallbackLangValue option, we should load both the active language and the fallbackLangValue and merge them together. Then, we can clean some memory by removing the fallbackLangValue translation from the service's cache.

[NetanelBasal]

@Edouardbozon what's the status of the issue?

[edbzn]

@NetanelBasal I have something working, I have to add tests now. I will push it soon to get feedback.