Treat (translation) target language like domain and context

[malthe]

Provide translation domain, context and target language when doing implicit conversion.

Note that 'target_language' is no longer defined as a variable, but as internal state (and disallowed as a variable name) – similar to domain and context.

This fixes issue #369.

[coveralls]

Pull Request Test Coverage Report for Build 6879293751

• 15 of 15 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 87.488%

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Totals
Change from base Build 6877527967:	0.03%
Covered Lines:	4256
Relevant Lines:	4759

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💛 - Coveralls