Fix #45: follow the selected language for formula editing

docs/decisions.md

@@ -0,0 +1,12 @@
+# Decisions
+
+## Formula localization uses the existing interface preference
+
+PR #98 builds on the persisted `UiState.language` introduced by #31 and extended by #104.
+Español selects the Spanish function table and list/decimal separators; there is no competing
+locale setting. Formula syntax without a supplied translation table falls back to English.
+The editor translates to canonical English before engine commands, and translates back only
+for display. API, clipboard and XLSX syntax remain stable. Existing names override aliases.
+
+The revision is based directly on main after #31/#104 and Portuguese interface support (#82)
+merged. Those locales retain their existing formula fallback; this change adds only Spanish.

docs/formula-input.md

@@ -0,0 +1,38 @@
+# Formula language
+
+View → Interface language is the existing, persisted language preference. Choose **Español**
+for Spanish formula entry and display. On first run the preference follows the system language.
+English and the other interface languages currently use the English formula table and separators.
+
+| Spanish | Canonical English |
+| --- | --- |
+| `SUMA` | `SUM` |
+| `PROMEDIO` | `AVERAGE` |
+| `SI` | `IF` |
+| `CONTAR` | `COUNT` |
+| `SUMAR.SI` | `SUMIF` |
+| `CONTAR.SI` | `COUNTIF` |
+| `BUSCARV` | `VLOOKUP` |
+| `HOY` | `TODAY` |
+| `VERDADERO`, `FALSO` | `TRUE`, `FALSE` (literals or function calls) |
+
+Names are case-insensitive. Spanish formulas use decimal commas (`1,5`), semicolon arguments
+and reference unions, and backslashes between array columns (`{1\2;3\4}`). For example,
+`=SI(VERDADERO;SUMA(1,5;2,25);0)` evaluates to 3.75. English mode retains comma arguments,
+decimal points and comma array columns; it does not silently translate Spanish aliases.
+
+The cell editor, formula bar, Show Formulas, autocomplete, the Formula ribbon, Insert Function, and argument hints
+use the selected formula language. Common Spanish function help is translated; functions
+without a Spanish entry retain their English names/help. General interface labels and errors
+fall back to English where Spanish translations are not provided. An edit captures its language
+when it starts so switching the preference cannot reinterpret unfinished input.
+
+Workbook/sheet names, table names, LET bindings and LAMBDA parameters take precedence over
+aliases. Text, sheet names and table column names are unchanged. When a localized spelling
+would collide with a name, displayed formulas retain the canonical spelling to preserve meaning.
+
+Stored formulas, XLSX import/export, engine commands, control/API calls, CLI evaluation and
+clipboard exchange retain strict English syntax. This also applies to advanced dialogs such as
+Name Manager and conditional-format rules. Localization happens at the cell/formula-bar editor
+boundary, including filling the selected range with Ctrl+Enter; it does not change the workbook
+format or teach the calculation engine ambiguous aliases.