My middle kid had some similar characteristics to those described in the original post, though not everything matches. She mixed up words, mainly abstract types of things. I should have kept track of what she mixed up, but the most notable mix-up was hot and cold. Despite correcting her every time she mixed up, the hot-cold thing continued until she was at least six.
She had some speech issues - articulation - and went through speech therapy at age three and then again at age eight. At age two/three, she would often say only the second syllable of a two syllable word, or the third and fourth of a four syllable word. While I didn't really pay to much attention to them, elementary reading assessments seemed to put her at a lower level than her true level. She would substitute for some words and start/stop (not fluid) reading when asked to read out loud. I see my youngest do that too with both reading and speech, and one teacher called it stuttering (which it isn't - more of hesitation). Lexile level assessments for middle kid which did not involve reading out loud (ie MAP testing) yielded much better results.
At 14 (15 in a few weeks), she still will sometimes ask a definition of a word that she should know (but a word you seldom would use). Often these are words that her eight year old sister knows. I did ask a psychologist at an established local child study center (center has been there at least 50 years) and she did suggest possible reasons for the language issue. However the testing seemed expensive, and she was in middle school by that time, and no issues in school.
She had some sensory type issues as well such as tags on clothes and food texture. We never looked for any help with those. We tolerated them for a while, then as she got older, we just told her to "grow up".
She has always done well on aptitude and achievement tests, and shows very "even" scores across subtests, so no one ever mentioned possible language issues, though some stuff she did was a bit odd.
And just an interesting similarity to the OPs case, all three of my kids attended a French immersion school (youngest still there, just started 3rd grade). We think the immersion has helped all three of them, and we don't think any issues we have seen are related to foreign language immersion.
