We say 'Fetch!' as one-word command to a dog, because it's clear, simple and unambiguous. It's not impolite to say 'Fetch your pencil' - it's just unusual, and can sound a little old-fashioned.
If you're in a particular location with someone, and you want them to go somewhere (perhaps to another part of the room or into another room), get their laptop and then return with the laptop, it would be most natural to say either
Go get your laptop (US English)
Go and get your laptop (GB English)
'Fetch your laptop' is possible, but less common. We would never say 'Go seek your laptop'. You can't translate 'Va chercher' into English in that way. It sounds very strange indeed.
And no, 'seek' and 'fetch' don't have the same meaning:
'Fetch' means 'go and get', as in the examples above.
'Seek' doesn't mean 'fetch'. 'Seek' means 'look for' - as in 'try to find' - but it is a relatively rare word in everyday modern English. Most native English speakers can go for years without ever saying this word at all. You can happily forget about the word 'seek' if you like - it really isn't useful or common.
Finally, you can't say 'Thank you for lighting me up' unless you are a Christmas tree!