Im having a similar problem but it is shipping related and not language related. Any advise would be greatly appriciated.
Resource string (plugins.shipping.fixedrateshipping.fields.rate) is not found. Language ID = 1 Resource string (plugins.shipping.fixedrateshipping.fields.shippingmethodname) is not found. Language ID = 1
This is making the plugin not work correctly. It displays on checkout but has no value to the method even tho is has been set and does not get added to the final checkout price.
In Admin > Configuration > Plugins, Uninstall "Fixed Rate Shipping", and then Install it. You will lose your config, so be sure to write down before and re-configure after you re-install.
I did what you said i should do many times to try fix the error but with no success. When i re install all the setting are still there it doesn't clear any config. Please see attached pic of checkout what its doing.