Support separate lists for origin and target languages in chromium translation engine

[atuchin-m]

For internal translation implementation we will use chromium components/translate engine.
MVP-version will use Brave backend that supports a limited number of translate pairs (i.e de => en).
To work under these restrictions we need to implement two separate language lists (one for source and once for target).

Note: all the changes should be done under the disabled compile flag enable_brave_translate_go, therefore nothing should be changed for customers.

for QA:

1. this issue can't be checked before the buildflag will be enabled (in related issues)
2. check that all settings UI in bubble works well for different languages (Refer to the "Supported language pairs" section in the Brave Translate MiniSpec)

[atuchin-m]

#18593

[stephendonner]

@atuchin-m mind pointing us (QA) to the Brave Translate MiniSpec you referenced? Also, some step-by-step instructions would be great, as I don't think many of us are familiar with this feature, much less the changes made; thanks!

[atuchin-m]

@stephendonner
The key changes here have been done under a disabled buildflag enable_brave_translate_go.
So better to check this issue and https://github.com/brave/brave-browser/issues in combination.

For the MiniSpec see a comment here

[atuchin-m]

The code is removed in #25368. We can return it as soon as we need it.