[Fix] Allow to use multiple translation functions with different namespaces and keyPrefixes

[wilhelmhb]

Why am I submitting this PR

Using multiple translation functions is currently a pain point for me.

Does it fix an existing ticket?

Yes #494 Also provides a workaround for #973 Should also fix #737

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [x] tests are included and pass: yarn test (see details here)
• [x] documentation is changed or added (not relevant IMHO)