Apertium is a free/open-source platform for developing rule-based machine translation systems!
To try Apertium
https://apertium.org is the official site, and offers all the released language pairs Alternative installations:
http://aplica.prompsit.com/ – Prompsit is a company heavily involved in development of the Apertium platform, and also offers a simple web interface to the released language pairs https://www.softcatala.org/traductor/ – Softcatalà works on Catalan localisation and offers the Catalan←→* language pairs here http://jorgal.uit.no/ – the University of Tromsø works on Saami language pairs; this site runs the latest in-development version of Northern Saami→Norwegian Bokmål http://gtweb.uit.no/tolkimine/ – another site by University of Tromsø, the latest in-development version of Estonian←→Finnish https://softaragones.org/traductor/ - Apertium Luengas d'Aragón - Softaragonés offers here the language pairs between the three native languages of Aragon: Aragonese - Catalan - Spanish
There are several applications which work from the desktop without full installation. The easiest solution for Windows and Mac users is Apertium Simpleton UI. You can install translator packages from within that program (though only the pairs that are on the website, not the "in development" pairs).