Despite being the same languages, Japanese, Chinese and other Asian languages are differently used (or have different characteristics) in caricatures in various Countries. Why is this so?
This is a better question for /r/linguistics, but the basic rundown is that /l/ and /r/ are liquid consonants and English just so happens to use both of them. Korean only has the single ㄹ sound which is in the middle of the two sounds. The result is that Koreans will frequently mix the two sounds up, especially if they learn English through reading - the words "rob" and "lob" would both be written as 랍 or 롭, depending on which vowel sound you want to use. German has different phonology and as a result has a different shift of sound.
It should be noted that Japanese, Chinese, and Korean have different phonological rules and as such cause different problems for English L2 speakers. In Korean, the l/r sound can change based on the location in the word. A word that starts with the sound is generally neutral, while a word like 나라, (nara - earth)where the /r/ sound is on the second syllable sounds like an /r/. A word like 전라도 (Jeolla-do, a province) has more of an /l/ sound. In Chinese, they have the opposite issue.
Anyway, unless you want a history of Asian language romanization and how sound representations have changed, go to /r/linguistics for a better answer.
I'm not 100% sure I understand your question: Are you asking "why do English people stereotype Asians as pronouncing L as R when speaking English, whereas Germans stereotype Asians as pronouncing R as L when speaking German?"