Create an approrate Quick Template, at the options tab enable the option 'Use for reply messages', next time you reply to a message, you select the down arrow next to the reply button and you can select the proper template from a list.
Same goes for new messages.

There's also a possibility to automate it more or less, but this will work only for contacts in your addrress book. Use one of the fields in the AB that you don't use as a language indicator.
Let's take the fax entry.
Use 'eng' as code for the English speaking contacts and 'pol' for let's say Polish speaking contacts.
Create quick templates for both English and Polish messages and keep the names meaningful, like neweng, newpol, replyeng and replypol.
Now you've got to decide what language you want to use as default, the best way to decide that is to use the langauge you use most for contacts not in your AB.
Let's take the Polish option again.
Change the new message template for your account into something like this:
%If:"%ABToFax"="Eng":"%QInclude('neweng')":"%QInclude('newpol')"
or something like this:
%QInclude('new%ABToFax="pol"')
Both macros have the same result, the if macro give you more freedom with naming your templates, as the name for your quick template isn't built from several parts, the other macro offers the option to expand your language set, simply by adding a third set of templates and a language id like dut for Dutch.
Your reply templates should work the same way, just substitute new with reply in the relevant parts of the macros.

The quick templates you're using should best begin with the %Clear macro, so if you want to overrule your address book based selection you simply type replypol Ctrl-space and the Polish reply template will be used while automatically clearing the template that was used till then. Note that you should do that before typing your own message, as your own text will be erased by the %Clear macro too.

Have fun.