Try to avoid use of the word 'got' (gotten) - they are 'uneducated' English. 'got' is used as a universal substitute by the inarticulate.
I got really fluent in German being here just after one year.
I've gotten really fluent in German being here just after one year.
I've become really fluent in German after being here for only a year.
"I became really fluent in German in just one year of being here."
I've become really fluent in German in just one year of being here.
"I became really fluent ..."
Note that when you use 'after' with the past tense verb, that is, "I became...after one year/after a year", it suggests this fluency suddenly occurred at the end of that year, whereas the meaning is, that your fluency increased over the course of that year, so that by the end of that year, you considered you were 'really fluent'.
e.g. I was promoted to Supervisor after only one year of working there.
- For Teachers