Rework language pair data structure to be more efficient with large numbers of supported pairs

[trevorparscal]

Google Translate, for example, uses intermediate languages internally to provide at least basic translation support for all 104 languages, making the list of translation pairs 104²-104. This could be done more efficiently.

[santhoshtr]

May be take a look at how cxserver does this? https://github.com/wikimedia/mediawiki-services-cxserver/blob/master/config/transform.js based on https://github.com/wikimedia/mediawiki-services-cxserver/blob/master/config/Google.yaml

(I am not sure about the duplication of features/code, still..)