If you're planning to be a teacher, you need to have completed your stay abroad before submitting your M.Ed. thesis. This seems a long way away when you first start studying, but it can take time to find a suitable opportunity or to apply for exchange and teaching programmes.
It is important to note that students are usually 'beurlaubt' during their stay abroad, and thus do not lose any time completing the degree. According to the Prüfungsordnung, 'der studienrelevante Auslandsaufenthalt ist kein Bestandteil der Regelstudienzeit.'
Generally speaking, the most convenient point for a period spent studying abroad in an English-speaking country is either the middle semesters of the bachelor degree (semester four, the summer between semesters four and five, or semester five in particular) or the time immediately after the completion of the first degree. (A popular option is to go abroad for a period of six months to a year between the BA and MEd degrees.)
Given the early application deadlines for most exchange programms, semester five is the best option for commencing a period of study abroad - due to overlapping semesters it is rarely practical to start university in the summer semester in other countries, although this can work in years when the Oldenburg semester finishes in late January, and is a good option for Australia and South Africa due to the different academic year in the southern hemisphere!
In your planning, you should also keep the scheduling of compulsory internships (Orientierungspraktikum/ASP/FP/FEP/Praxissemester) in mind. If you are abroad in September it is frequently possible to complete a compulsory internship in January instead - contact the DIZ for further information.