I would say it depends on your timeframe and finances. If you have the time and can afford it, a CELTA is a solid, if pricey, intro to ESL theory and practice--if you have a spare month. Otherwise there are plenty of shorter, cheaper qualifications which you can get which may however give you less of a preparation.
If you do commit to living in Germany, learning at least elementary German should be a priority: it shows respect for your host country, it will help you get much better jobs, and it will give you some insight into what your students must go through.
- For Teachers