One keyboard two languages on Ubuntu 17.10

Solution 1:

The US International keyboard layout isn't designed for diacritics because the target language is English and English doesn't use diacritics (it should, at least for foreign words like fiancé or coupé, but it doesn't and it sometimes results in an incorrect pronunciation like "coupe" instead of "coupeh" - I'm looking at you US citizens, in the other side of the pound we do pronounce it correctly).

In order to use input languages requiring diacritics users must choose the "US International with dead keys" variant, with caveats:

• The apostrophe key is the same for the acute diacritic symbol. With "dead keys" you must press SPACE after the key.
• As above, quotation marks is used for diaeresis (umlaut) therefore if you want the quotation mark instead, you need to press SPACE after the key.
• As a general rule all the characters with diacritics are produced by a sequence of two keys, first the diacritic symbol then the letter - Example: à is obtained by pressing ` followed by a - the same way used with French, Spanish (and others) keyboard layouts.
• Other special characters aren't so obvious as the example above. For instance, for the character ç, used in Portuguese and Catalan, there are two ways: 1. AltGr+c or 2. ' folowed by c.

Solution 2:

For a newly created account it might happen that the current input source is changed undesirably if you change the (display) language. However, once you have switched input source explicitly once or twice, the selected (display) language and the current input source should be mutually independent.

Consequently, the answer to your question is yes. You can have any combination of (display) language and input source.