Another idea
To achieve your goal you could even use the following script ingame into the chat window and press return key.
Attention:
The game will automatically reload the UI afterwards!
This will change the texts to English. The videos and sounds should stay German if you are using the German client afaik.
Code:
/script SetCVar("language.2","en")
You can exchange the en with one of the following language variables:
en English
de German
fr French
jp Japanese (afaik only possible if you got the japanese client installed!)
es Spanish (unofficially language which might need "a Spanish translation addon" to work)
ru Russian (unofficially language which might need "a Russian translation addon" to work)
Back to your question
Which texts are still German? Maybe addon texts, or base game texts?
You definately need to do a /reloadui so the game swaps the texts as they won't be changed during gameplay!
You can ues the function
in lua to do a reloadui from your addon as well.