How do you say welcome in Lingala?

you can say don't mention it or most appreciated

ni hao (Chinese)

"Tervetuloa [PLACE]+[END]." Enter the name of the place to the [PLACE]. But the [END] is very difficult to foreigners. You have to say the right kind of syllable at the end. These are combined...

welcome abourd or welcome onboard is both good because the both make sence

No, You should say "Your Welcome"