Correcting Coordinate Display Errors in FSX Flight Plans

The issue you're encountering with coordinates being reset to 0º, 0º is intriguing and certainly a bit of a navigational conundrum. From the snippets you've shared, it seems the degree symbol is being misinterpreted, causing a default to the zero coordinate, which is the latitudinal and longitudinal intersect in the Gulf of Guinea – often humorously referred to as 'Null Island' in geospatial circles.

Considering your thorough approach to troubleshooting thus far – language settings, FSX.cfg regeneration, and scenery library rebuilding – it appears you've covered the fundamental bases. Your intuition about the encoding is spot-on. UTF-8 encoding should normally support the degree symbol, but FSX has some quirks, especially regarding how it interfaces with different system locales and language settings.

Here are a few additional avenues to explore:

1. Ensure that the language for non-Unicode programs is set to English (United States) within the Control Panel's Regional and Language settings. FSX can be particular about system locale.
2. Examine the FSX.cfg file for any anomalies or corrupt entries that might have gone unnoticed. Manual editing can sometimes introduce non-visible characters that may cause issues.
3. If you're comfortable with a hex editor, you can open the PLN file and manually check if the degree symbol is correctly encoded. It should appear as C2 B0 in UTF-8 hex.
4. Since you've mentioned using accents for mission files, check if those specific files are also saved in UTF-8 without BOM (Byte Order Mark), which FSX may not interpret correctly.
5. It may be worthwhile to create a new flight plan using the FSX Steam Edition flight planner and save it to inspect the encoding format and symbols used, comparing it to one saved from the non-Steam version.

Furthermore, you could consider using a dedicated flight planning tool that is compatible with FSX Steam Edition to create and save your flight plans, then import them into FSX. Tools like Little Navmap or Plan-G can be quite adept at handling various encoding issues and might offer a workaround to this problem.

Lastly, while it seems unrelated, the correlation between the non-Steam and Steam versions could be indicative of how the two different applications handle user settings and system integration. It's possible that the non-Steam version was more forgiving with encoding discrepancies or system locale settings.

If the issue persists, I would be keen to delve further into the specifics. Please share a bit more detail about your operating system settings, especially regional settings, and any specificities in the file paths or directories where the PLN and FLT files are saved.

With a bit more context, we can navigate towards a resolution that gets your waypoints accurately plotted and your virtual flights back on the proverbial radar.

Fly high and all the best.