Nice to have: Option to keep English original keywords as valid

[aaltat]

When we have translated keywords and the original English keywords, the test run would fail because the English keywords are unknown.
Would be nice to have an option to make this current behavior as a strict mode, and have the option to allow mixed translation and English keywords the default.

For example:

mode=multi   # The default (or mixed)

or

mode=strict   # The exact translation (or single) (or exact) ....

Copy of MarketSquare/robotframework-seleniumlibrary-translation#59 because it also needs support from PLC.