So you can think of iść (going/walking right now) or pójść as:
• Ja idę (I am going) -> focus on the process going right now (i.e. you're going to the supermarket right now)
• Ja pójdę (I will go) -> focus on finishing going somewhere (i.e. you are speaking about going to Germany - as your destination)
• Ja szedłem (I was going) -> focus on the process of walking (i.e. speaking about the time when you walked between home and work)
• Ja poszedłem (I went) -> focus on finished the action (i.e. speaking about that you came to Germany - and now youre staying here.)
What about the verb 'chodzić' (to go/walk regularly/habitually) - are there also perfective and past tenses?
Your understanding of the tenses and aspects seems to be correct, however this verb isn't used for going to another country in order to stay there, since it implies waking on foot. The only context can imagine to find "idę do Niemiec" is if you live in Zgorzelec/Gubin/Słubice/Kostrzyn and cross the border on foot to buy some groceries.
Chodzić has a past and present form. (chodziłem/chodzę).
You can make it perfective by adding po-, but this word will mean to originate / to come from in over 95% of contexts. Otherwise it means either "to walk around for a short while" or "to go somewhere regularly, but not very often". So, it's not a "real perfective" form.
Can I correct a few small things? :)
I understand "tylko jako ciekawość" and I guess it's correct grammatically, but I think that the most common way of saying that is "Tak z ciekawości" (just out of curiosity). Then "angielskie wyrażenie" (neuter, plus small letter for an adjective describing the nationality). "To często znaczy/oznacza" (oznaczy is Future Tense). And then "beż" is the colour beige, you meant "bez" :) But even with those few mistakes, good job!
No, I wouldn't translate "I went to answer it" as "poszedłem" if I didn't need to walk anywhere. I'd just say "I answered".
Just in case, "to answer the phone" is "odebrać telefon" (just "odebrać" will probably be enough if the context is obvious).
No pewnie! Zawsze jestem wdzięczny za pomoc. Bez popraw nie ma nauki :)
Tak dla jasności; angielskie wyrażenie w kontekście o co mi chodzi, zwykle znaczy, że ten czyn nie staje się skończony. W przykładzie o telefonie, dzwonienie się zatrzymał zanim mogłem odebrać, może tylko sięgnąłem po komórkę.
Może w tym kontekście "I went to...." po polsku znaczy "miałem zamiar"? :)