HI Juan Manuel,
you are absolutely right. There are many things that should improve in the LaTeX exporter and simplifying language treatment is one of them. What I show in my example is how to circumvent all the limitations in the exporter and give a working example for people who need it.
Adding AUTO to latex_class_options would be extremely helpful, since then you can load babel indicating the main language only and even avoid the PassOptions and can live with \usepackage{alphabeta} only.
This is a MWE for mixing German and Greek which I use to compare with what org-mode generates:
---cut---
\documentclass[greek,ngerman]{scrartcl}
%% \documentclass[greek,ngerman]{scrreprt}
%% \documentclass[greek,ngerman]{scrbook}
%% \documentclass[greek,ngerman]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
%% \PassOptionsToPackage{main=ngerman}{babel}
\usepackage{alphabeta}
\begin{document}
Ach, unsere Freunde die Griechen: \textgreek{νους} (mit \verb|\textgreek{}|).\par
Ach, unsere Freunde die Griechen: νους (direkt im Text).
\end{document}
---cut---
The commented document classes are all I had the time to test.
Best, /PA