Go to Settings > Preferences > Language, then move the unused languages to the “Disabled Items” side (docs). This has the primary effect of hiding those language types from the Language menu, but it also has the secondary effect of removing those languages from the SaveAs list as well, which is what you want.

Also, if you select the Language entry before doing the Save/SaveAs action, it will automatically select the right entry from that drop-down.

Altnerately, if you have Settings > Preferences > MISC > Set save dialog extension to *.*, then you can just type the extension you want, and that extension will be applied and Notepad++ will automatically assign that file to the right Language menu entry, just like when you load a file with that extension.