[tobeportable]

You should 100% have a look at text-expansion tools e.g. espanso with https://hub.espanso.org/packages/french-accents/.

The general idea is that you would type something like :e' to type out é. This allows you to use a single keyboard layout across languages.

[ferzul]

which is helpful if the three languages are russian, english and japanese

[tobeportable]

You are right, this method only works for specific set of languages, e.g same family languages.