How can someone speak their first language/mother tongue with an accent from another language?

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I think if we are exposed a lot to a language, it will inevitably impact the way we speak other languages. Also we shouldn’t forget about the affective factor that also plays a great role in how both receptive and productive language skills are developed. My point is strong emotions/feelings we experienced while being exposed to a language will likely influence one or several language competency aspects. It’s possible that you’ve had a special bond with your grandpa and/or have had some vivid memories related to interacting with him which impacted some of your pronunciation aspects of your mother tongue. You might also have a special affiliation/liking/preference for the German culture and/or language since you’re saying that “German sounds come easier to me than Swedish”. Hope that helps.