Allow to change the current language

@Kobe Wright can you edit the MR to point to 10.1.x?

@tim.plunkett Sure! Created a new MR for 10.1.x

Even though this is a task seems like a feature request.

This will need test coverage of some kind. Probably simple kernel test?

Will need a change record to announce new function.

Left some NIT comments in MR 3451

Will need tests also. Not sure if subsystem maintainer has any thoughts.

sleitner → closed merge request !3630

Rerolled to 11.x and added language reset. Please review.

I don't have explicitly proof but I have feelings about this probably causing some problem with caching. I think for cache contexts related to the already negotiated language, it would cache the strings / render outputs generated from the strings in the wrong language. Would be great to have someone with deep experience of the cache system consider this. In other words I think this is meant to be negotiated and used from then on, and not changed.

The language manager has the ability to set a configuration override language. That works for example for user_mail(), where we just need to load configuration in the language of the user.

However, more advanced cases need to be able to change the current language completely. For example Simplenews, where we are building newsletter issues, that includes viewing entities, t() calls in e.g. templates, views queries that use the language context.. all kinds of things.

What makes it not possible to send along the language explicitly to those operations like "user_mail()" does?

Thanks @Gábor Hojtsy with sign off think we could use the CR now