I'm going to do a series of posts on how to set up your PC or mobile device to type Vietnamese with the diacritics. You should be able to type người nước ngoại, not nguoi nuoc ngoai or ngu'o'\i nu'o'/c ngoa.i. The latter is hideous and the blank one is unreadable to anyone but a native speaker.
First, you don't need to install any special fonts. All personal computers and almost all devices have extended character sets that include Vietnamese and even CJK (Chinese-Japanese-Korean). They're already on your device. If you have a phone restricted to a small set of languages, this won't help.
For Linux or any unlisted device, ask me, or ask someone who has one because I don't know all of them. I do type Vietnamese on Linux but there may be easier ways than what I use.